diff --git a/lib/connection.js b/lib/connection.js
index d8e60bd6e00..31c35af3ee4 100644
--- a/lib/connection.js
+++ b/lib/connection.js
@@ -753,18 +753,6 @@ Connection.prototype.openUri = function(uri, options, callback) {
     });
   });
 
-  const _handleReconnect = () => {
-    // If we aren't disconnected, we assume this reconnect is due to a
-    // socket timeout. If there's no activity on a socket for
-    // `socketTimeoutMS`, the driver will attempt to reconnect and emit
-    // this event.
-    if (_this.readyState !== STATES.connected) {
-      _this.readyState = STATES.connected;
-      _this.emit('reconnect');
-      _this.emit('reconnected');
-    }
-  };
-
   const promise = new Promise((resolve, reject) => {
     if (_this.client != null) {
       _this.client.close();
@@ -778,111 +766,9 @@ Connection.prototype.openUri = function(uri, options, callback) {
         return reject(error);
       }
 
-      const db = dbName != null ? client.db(dbName) : client.db();
-      _this.db = db;
-
-      // `useUnifiedTopology` events
-      const type = get(db, 's.topology.s.description.type', '');
-      if (options.useUnifiedTopology) {
-        if (type === 'Single') {
-          const server = Array.from(db.s.topology.s.servers.values())[0];
-
-          server.s.topology.on('serverHeartbeatSucceeded', () => {
-            _handleReconnect();
-          });
-          server.s.pool.on('reconnect', () => {
-            _handleReconnect();
-          });
-          client.on('serverDescriptionChanged', ev => {
-            const newDescription = ev.newDescription;
-            if (newDescription.type === 'Standalone') {
-              _handleReconnect();
-            } else {
-              _this.readyState = STATES.disconnected;
-            }
-          });
-        } else if (type.startsWith('ReplicaSet')) {
-          client.on('topologyDescriptionChanged', ev => {
-            // Emit disconnected if we've lost connectivity to _all_ servers
-            // in the replica set.
-            const description = ev.newDescription;
-            const servers = Array.from(ev.newDescription.servers.values());
-            const allServersDisconnected = description.type === 'ReplicaSetNoPrimary' &&
-              servers.reduce((cur, d) => cur || d.type === 'Unknown', false);
-            if (_this.readyState === STATES.connected && allServersDisconnected) {
-              // Implicitly emits 'disconnected'
-              _this.readyState = STATES.disconnected;
-            } else if (_this.readyState === STATES.disconnected && !allServersDisconnected) {
-              _handleReconnect();
-            }
-          });
-
-          db.on('close', function() {
-            const type = get(db, 's.topology.s.description.type', '');
-            if (type !== 'ReplicaSetWithPrimary') {
-              // Implicitly emits 'disconnected'
-              _this.readyState = STATES.disconnected;
-            }
-          });
-        }
-      }
-
-      // Backwards compat for mongoose 4.x
-      db.on('reconnect', function() {
-        _handleReconnect();
-      });
-      db.s.topology.on('reconnectFailed', function() {
-        _this.emit('reconnectFailed');
-      });
-
-      if (!options.useUnifiedTopology) {
-        db.s.topology.on('left', function(data) {
-          _this.emit('left', data);
-        });
-      }
-      db.s.topology.on('joined', function(data) {
-        _this.emit('joined', data);
-      });
-      db.s.topology.on('fullsetup', function(data) {
-        _this.emit('fullsetup', data);
-      });
-      if (get(db, 's.topology.s.coreTopology.s.pool') != null) {
-        db.s.topology.s.coreTopology.s.pool.on('attemptReconnect', function() {
-          _this.emit('attemptReconnect');
-        });
-      }
-      if (!options.useUnifiedTopology || !type.startsWith('ReplicaSet')) {
-        db.on('close', function() {
-          // Implicitly emits 'disconnected'
-          _this.readyState = STATES.disconnected;
-        });
-      }
-
-      if (!options.useUnifiedTopology) {
-        client.on('left', function() {
-          if (_this.readyState === STATES.connected &&
-              get(db, 's.topology.s.coreTopology.s.replicaSetState.topologyType') === 'ReplicaSetNoPrimary') {
-            _this.readyState = STATES.disconnected;
-          }
-        });
-      }
-
-      db.on('timeout', function() {
-        _this.emit('timeout');
-      });
-
-      delete _this.then;
-      delete _this.catch;
-      _this.readyState = STATES.connected;
-
-      for (const i in _this.collections) {
-        if (utils.object.hasOwnProperty(_this.collections, i)) {
-          _this.collections[i].onOpen();
-        }
-      }
+      _setClient(_this, client, options, dbName);
 
       resolve(_this);
-      _this.emit('open');
     });
   });
 
@@ -916,6 +802,125 @@ Connection.prototype.openUri = function(uri, options, callback) {
   return this;
 };
 
+function _setClient(conn, client, options, dbName) {
+  const db = dbName != null ? client.db(dbName) : client.db();
+  conn.db = db;
+  conn.client = client;
+
+  const _handleReconnect = () => {
+    // If we aren't disconnected, we assume this reconnect is due to a
+    // socket timeout. If there's no activity on a socket for
+    // `socketTimeoutMS`, the driver will attempt to reconnect and emit
+    // this event.
+    if (conn.readyState !== STATES.connected) {
+      conn.readyState = STATES.connected;
+      conn.emit('reconnect');
+      conn.emit('reconnected');
+    }
+  };
+
+  // `useUnifiedTopology` events
+  const type = get(db, 's.topology.s.description.type', '');
+  if (options.useUnifiedTopology) {
+    if (type === 'Single') {
+      const server = Array.from(db.s.topology.s.servers.values())[0];
+      server.s.topology.on('serverHeartbeatSucceeded', () => {
+        _handleReconnect();
+      });
+      server.s.pool.on('reconnect', () => {
+        _handleReconnect();
+      });
+      client.on('serverDescriptionChanged', ev => {
+        const newDescription = ev.newDescription;
+        if (newDescription.type === 'Standalone') {
+          _handleReconnect();
+        } else {
+          conn.readyState = STATES.disconnected;
+        }
+      });
+    } else if (type.startsWith('ReplicaSet')) {
+      client.on('topologyDescriptionChanged', ev => {
+        // Emit disconnected if we've lost connectivity to _all_ servers
+        // in the replica set.
+        const description = ev.newDescription;
+        const servers = Array.from(ev.newDescription.servers.values());
+        const allServersDisconnected = description.type === 'ReplicaSetNoPrimary' &&
+          servers.reduce((cur, d) => cur || d.type === 'Unknown', false);
+        if (conn.readyState === STATES.connected && allServersDisconnected) {
+          // Implicitly emits 'disconnected'
+          conn.readyState = STATES.disconnected;
+        } else if (conn.readyState === STATES.disconnected && !allServersDisconnected) {
+          _handleReconnect();
+        }
+      });
+
+      db.on('close', function() {
+        const type = get(db, 's.topology.s.description.type', '');
+        if (type !== 'ReplicaSetWithPrimary') {
+          // Implicitly emits 'disconnected'
+          conn.readyState = STATES.disconnected;
+        }
+      });
+    }
+  }
+
+  // Backwards compat for mongoose 4.x
+  db.on('reconnect', function() {
+    _handleReconnect();
+  });
+  db.s.topology.on('reconnectFailed', function() {
+    conn.emit('reconnectFailed');
+  });
+
+  if (!options.useUnifiedTopology) {
+    db.s.topology.on('left', function(data) {
+      conn.emit('left', data);
+    });
+  }
+  db.s.topology.on('joined', function(data) {
+    conn.emit('joined', data);
+  });
+  db.s.topology.on('fullsetup', function(data) {
+    conn.emit('fullsetup', data);
+  });
+  if (get(db, 's.topology.s.coreTopology.s.pool') != null) {
+    db.s.topology.s.coreTopology.s.pool.on('attemptReconnect', function() {
+      conn.emit('attemptReconnect');
+    });
+  }
+  if (!options.useUnifiedTopology || !type.startsWith('ReplicaSet')) {
+    db.on('close', function() {
+      // Implicitly emits 'disconnected'
+      conn.readyState = STATES.disconnected;
+    });
+  }
+
+  if (!options.useUnifiedTopology) {
+    client.on('left', function() {
+      if (conn.readyState === STATES.connected &&
+          get(db, 's.topology.s.coreTopology.s.replicaSetState.topologyType') === 'ReplicaSetNoPrimary') {
+        conn.readyState = STATES.disconnected;
+      }
+    });
+  }
+
+  db.on('timeout', function() {
+    conn.emit('timeout');
+  });
+
+  delete conn.then;
+  delete conn.catch;
+  conn.readyState = STATES.connected;
+
+  for (const i in conn.collections) {
+    if (utils.object.hasOwnProperty(conn.collections, i)) {
+      conn.collections[i].onOpen();
+    }
+  }
+
+  conn.emit('open');
+}
+
 /*!
  * ignore
  */
@@ -1330,6 +1335,56 @@ Connection.prototype.optionsProvideAuthenticationData = function(options) {
       ((options.pass) || this.authMechanismDoesNotRequirePassword());
 };
 
+/**
+ * Returns the [MongoDB driver `MongoClient`](http://mongodb.github.io/node-mongodb-native/3.5/api/MongoClient.html) instance
+ * that this connection uses to talk to MongoDB.
+ *
+ * ####Example:
+ *     const conn = await mongoose.createConnection('mongodb://localhost:27017/test');
+ *
+ *     conn.getClient(); // MongoClient { ... }
+ *
+ * @api public
+ * @return {MongoClient}
+ */
+
+Connection.prototype.getClient = function getClient() {
+  return this.client;
+};
+
+/**
+ * Set the [MongoDB driver `MongoClient`](http://mongodb.github.io/node-mongodb-native/3.5/api/MongoClient.html) instance
+ * that this connection uses to talk to MongoDB. This is useful if you already have a MongoClient instance, and want to
+ * reuse it.
+ *
+ * ####Example:
+ *     const client = await mongodb.MongoClient.connect('mongodb://localhost:27017/test');
+ *
+ *     const conn = mongoose.createConnection().setClient(client);
+ *
+ *     conn.getClient(); // MongoClient { ... }
+ *     conn.readyState; // 1, means 'CONNECTED'
+ *
+ * @api public
+ * @return {Connection} this
+ */
+
+Connection.prototype.setClient = function setClient(client) {
+  if (!(client instanceof mongodb.MongoClient)) {
+    throw new MongooseError('Must call `setClient()` with an instance of MongoClient');
+  }
+  if (this.client != null || this.readyState !== STATES.disconnected) {
+    throw new MongooseError('Cannot call `setClient()` on a connection that is already connected.');
+  }
+  if (!client.isConnected()) {
+    throw new MongooseError('Cannot call `setClient()` with a MongoClient that is not connected.');
+  }
+
+  _setClient(this, client, { useUnifiedTopology: client.s.options.useUnifiedTopology }, client.s.options.dbName);
+
+  return this;
+};
+
 /**
  * Switches to a different database using the same connection pool.
  *
diff --git a/test/connection.test.js b/test/connection.test.js
index f259e6d590f..e0a81a31ee1 100644
--- a/test/connection.test.js
+++ b/test/connection.test.js
@@ -10,6 +10,7 @@ const Promise = require('bluebird');
 const Q = require('q');
 const assert = require('assert');
 const co = require('co');
+const mongodb = require('mongodb');
 const server = require('./common').server;
 
 const mongoose = start.mongoose;
@@ -1185,4 +1186,20 @@ describe('connections:', function() {
       assert.equal(db2.config.useCreateIndex, true);
     });
   });
+
+  it('allows setting client on a disconnected connection (gh-9164)', function() {
+    return co(function*() {
+      const client = yield mongodb.MongoClient.connect('mongodb://localhost:27017/mongoose_test', {
+        useNewUrlParser: true,
+        useUnifiedTopology: true
+      });
+      const conn = mongoose.createConnection().setClient(client);
+
+      assert.equal(conn.readyState, 1);
+
+      yield conn.createCollection('test');
+      const res = yield conn.dropCollection('test');
+      assert.ok(res);
+    });
+  });
 });