diff --git a/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/WindowDoFnOperator.java b/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/WindowDoFnOperator.java
index 7baf193ef191..69d701e14570 100644
--- a/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/WindowDoFnOperator.java
+++ b/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/WindowDoFnOperator.java
@@ -32,6 +32,7 @@
 import org.apache.beam.sdk.values.TupleTag;
 import org.apache.beam.sdk.values.WindowingStrategy;
 import org.apache.flink.api.java.functions.KeySelector;
+import org.joda.time.Instant;
 
 /** Flink operator for executing window {@link DoFn DoFns}. */
 @SuppressWarnings({
@@ -81,6 +82,7 @@ public WindowDoFnOperator(
   public void open() throws Exception {
     stateInternals = new HashMap<>();
     inMemTimerInternals = new HashMap<>();
+    setBundleFinishedCallback(this::onFinishBundle);
     super.open();
   }
 
@@ -128,8 +130,10 @@ protected DoFn<KeyedWorkItem<K, InputT>, KV<K, OutputT>> getDoFn() {
       // element or by the key of a firing timer
       stateInternalsFactory = key -> (StateInternals) keyedStateInternals;
     } else {
+      // On a non-keyed stream, no flink does not provide a state backend.
+      // We use an in memory state instead.
       stateInternalsFactory =
-          key -> stateInternals.computeIfAbsent(key, k -> InMemoryStateInternals.forKey(k));
+          key -> stateInternals.computeIfAbsent(key, InMemoryStateInternals::forKey);
     }
 
     TimerInternalsFactory<K> timerInternalsFactory;
@@ -137,6 +141,8 @@ protected DoFn<KeyedWorkItem<K, InputT>, KV<K, OutputT>> getDoFn() {
       // this will implicitly be keyed like the StateInternalsFactory
       timerInternalsFactory = key -> timerInternals;
     } else {
+      // On a non-keyed stream, no flink does not provide a timer service.
+      // We use an in memory timer instead.
       timerInternalsFactory =
           key -> inMemTimerInternals.computeIfAbsent(key, k -> new InMemoryTimerInternals());
     }
@@ -157,39 +163,44 @@ protected DoFn<KeyedWorkItem<K, InputT>, KV<K, OutputT>> getDoFn() {
     return doFn;
   }
 
-  @Override
-  void flushData() throws Exception {
+  private void advanceWatermarkAndFireTimers(Instant watermark) throws Exception {
     for (Map.Entry<K, InMemoryTimerInternals> entry : inMemTimerInternals.entrySet()) {
       K key = entry.getKey();
       InMemoryTimerInternals timer = entry.getValue();
-      timer.advanceInputWatermark(BoundedWindow.TIMESTAMP_MAX_VALUE);
-      timer.advanceProcessingTime(BoundedWindow.TIMESTAMP_MAX_VALUE);
-      timer.advanceSynchronizedProcessingTime(BoundedWindow.TIMESTAMP_MAX_VALUE);
+      timer.advanceInputWatermark(watermark);
+      timer.advanceProcessingTime(watermark);
+      timer.advanceSynchronizedProcessingTime(watermark);
 
       TimerData timerData = timer.removeNextEventTimer();
       while (timerData != null) {
         doFnRunner.processElement(
-            WindowedValue.valueInGlobalWindow(
-                KeyedWorkItems.timersWorkItem(key, Collections.singletonList(timerData))));
+                WindowedValue.valueInGlobalWindow(
+                        KeyedWorkItems.timersWorkItem(key, Collections.singletonList(timerData))));
         timerData = timer.removeNextEventTimer();
       }
 
       timerData = timer.removeNextProcessingTimer();
       while (timerData != null) {
         doFnRunner.processElement(
-            WindowedValue.valueInGlobalWindow(
-                KeyedWorkItems.timersWorkItem(key, Collections.singletonList(timerData))));
+                WindowedValue.valueInGlobalWindow(
+                        KeyedWorkItems.timersWorkItem(key, Collections.singletonList(timerData))));
         timerData = timer.removeNextProcessingTimer();
       }
 
       timerData = timer.removeNextSynchronizedProcessingTimer();
       while (timerData != null) {
         doFnRunner.processElement(
-            WindowedValue.valueInGlobalWindow(
-                KeyedWorkItems.timersWorkItem(key, Collections.singletonList(timerData))));
+                WindowedValue.valueInGlobalWindow(
+                        KeyedWorkItems.timersWorkItem(key, Collections.singletonList(timerData))));
         timerData = timer.removeNextSynchronizedProcessingTimer();
       }
     }
+  }
+
+  @Override
+  void flushData() throws Exception {
+    // equivalent to processPendingProcessingTimeTimers for in memory timers
+    advanceWatermarkAndFireTimers(new Instant(Long.MAX_VALUE));
     super.flushData();
   }
 
@@ -201,4 +212,12 @@ protected void fireTimer(TimerData timer) {
             KeyedWorkItems.timersWorkItem(
                 (K) keyedStateInternals.getKey(), Collections.singletonList(timer))));
   }
+
+  private void onFinishBundle() {
+      try {
+          advanceWatermarkAndFireTimers(new Instant(getEffectiveInputWatermark()));
+      } catch (Exception e) {
+          throw new RuntimeException(e);
+      }
+  }
 }