diff --git a/esrally/async_connection.py b/esrally/async_connection.py
index bd7e302a3..f5293d073 100644
--- a/esrally/async_connection.py
+++ b/esrally/async_connection.py
@@ -117,8 +117,10 @@ def perform_request(self, method, url, params=None, body=None, timeout=None, ign
         start = self.loop.time()
         response = None
         try:
-            with async_timeout.timeout(timeout or self.timeout.total, loop=self.loop):
-                response = yield from self.session.request(method, url, data=body, headers=headers)
+            request_timeout = timeout or self.timeout.total
+            with async_timeout.timeout(request_timeout, loop=self.loop):
+                # override the default session timeout explicitly
+                response = yield from self.session.request(method, url, data=body, headers=headers, timeout=request_timeout)
                 raw_data = yield from response.text()
             duration = self.loop.time() - start
 
diff --git a/esrally/client.py b/esrally/client.py
index e2df92d47..161d65013 100644
--- a/esrally/client.py
+++ b/esrally/client.py
@@ -161,6 +161,8 @@ async def on_request_end(session, trace_config_ctx, params):
         trace_config = aiohttp.TraceConfig()
         trace_config.on_request_start.append(on_request_start)
         trace_config.on_request_end.append(on_request_end)
+        # ensure that we also stop the timer when a request "ends" with an exception (e.g. a timeout)
+        trace_config.on_request_exception.append(on_request_end)
 
         # needs patching as https://github.com/elastic/elasticsearch-py-async/pull/68 is not merged yet
         class RallyAsyncTransport(elasticsearch_async.transport.AsyncTransport):
diff --git a/esrally/driver/runner.py b/esrally/driver/runner.py
index e832c0602..68a62d87e 100644
--- a/esrally/driver/runner.py
+++ b/esrally/driver/runner.py
@@ -605,12 +605,11 @@ async def __call__(self, es, params):
         except elasticsearch.TransportError as e:
             # this is caused by older versions of Elasticsearch (< 2.1), fall back to optimize
             if e.status_code == 400:
-                params = {"request_timeout": request_timeout}
                 if max_num_segments:
-                    await es.transport.perform_request("POST", "/_optimize?max_num_segments={}".format(max_num_segments),
-                                                       params=params)
+                    await es.transport.perform_request("POST", f"/_optimize?max_num_segments={max_num_segments}",
+                                                       timeout=request_timeout)
                 else:
-                    await es.transport.perform_request("POST", "/_optimize", params=params)
+                    await es.transport.perform_request("POST", "/_optimize", timeout=request_timeout)
             else:
                 raise e
 
diff --git a/tests/driver/runner_test.py b/tests/driver/runner_test.py
index 8370edc8a..840dc4eba 100644
--- a/tests/driver/runner_test.py
+++ b/tests/driver/runner_test.py
@@ -911,7 +911,7 @@ async def test_force_merge_override_request_timeout(self, es):
         es.indices.forcemerge.return_value = as_future()
 
         force_merge = runner.ForceMerge()
-        await force_merge(es, params={"index" : "_all", "request-timeout": 50000})
+        await force_merge(es, params={"index": "_all", "request-timeout": 50000})
 
         es.indices.forcemerge.assert_called_once_with(index="_all", request_timeout=50000)
 
@@ -934,7 +934,7 @@ async def test_optimize_with_defaults(self, es):
         force_merge = runner.ForceMerge()
         await force_merge(es, params={})
 
-        es.transport.perform_request.assert_called_once_with("POST", "/_optimize", params={"request_timeout": None})
+        es.transport.perform_request.assert_called_once_with("POST", "/_optimize", timeout=None)
 
     @mock.patch("elasticsearch.Elasticsearch")
     @run_async
@@ -944,8 +944,7 @@ async def test_optimize_with_params(self, es):
         force_merge = runner.ForceMerge()
         await force_merge(es, params={"max-num-segments": 3, "request-timeout": 17000})
 
-        es.transport.perform_request.assert_called_once_with("POST", "/_optimize?max_num_segments=3",
-                                                             params={"request_timeout": 17000})
+        es.transport.perform_request.assert_called_once_with("POST", "/_optimize?max_num_segments=3", timeout=17000)
 
 
 class IndicesStatsRunnerTests(TestCase):