diff --git a/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsAndCombineDoFn.java b/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsAndCombineDoFn.java
new file mode 100644
index 0000000000..d3eb50b226
--- /dev/null
+++ b/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsAndCombineDoFn.java
@@ -0,0 +1,114 @@
+/*
+ * Copyright (C) 2015 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package com.google.cloud.dataflow.sdk.util;
+
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.cloud.dataflow.sdk.transforms.Combine.KeyedCombineFn;
+import com.google.cloud.dataflow.sdk.transforms.windowing.BoundedWindow;
+import com.google.cloud.dataflow.sdk.values.KV;
+import com.google.common.collect.Maps;
+
+import org.joda.time.Instant;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Comparator;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.PriorityQueue;
+
+/**
+ * {@link GroupAlsoByWindowsDoFn} that uses combiner to accumulate input elements for non-merging
+ * window functions with the default triggering strategy.
+ *
+ * @param <K> key type
+ * @param <InputT> value input type
+ * @param <AccumT> accumulator type
+ * @param <OutputT> value output type
+ * @param <W> window type
+ */
+@SuppressWarnings("serial")
+public class GroupAlsoByWindowsAndCombineDoFn<K, InputT, AccumT, OutputT, W extends BoundedWindow>
+    extends GroupAlsoByWindowsDoFn<K, InputT, OutputT, W> {
+
+  private final KeyedCombineFn<K, InputT, AccumT, OutputT> combineFn;
+
+  public GroupAlsoByWindowsAndCombineDoFn(
+      KeyedCombineFn<K, InputT, AccumT, OutputT> combineFn) {
+    this.combineFn = combineFn;
+  }
+
+  @Override
+  public void processElement(ProcessContext c) throws Exception {
+    K key = c.element().getKey();
+    Iterator<WindowedValue<InputT>> iterator = c.element().getValue().iterator();
+
+    PriorityQueue<W> liveWindows =
+        new PriorityQueue<>(11, new Comparator<BoundedWindow>() {
+          @Override
+          public int compare(BoundedWindow w1, BoundedWindow w2) {
+            return Long.signum(w1.maxTimestamp().getMillis() - w2.maxTimestamp().getMillis());
+          }
+        });
+
+    Map<W, AccumT> accumulators = Maps.newHashMap();
+    Map<W, Instant> minTimestamps = Maps.newHashMap();
+
+    while (iterator.hasNext()) {
+      WindowedValue<InputT> e = iterator.next();
+
+      @SuppressWarnings("unchecked")
+      Collection<W> windows = (Collection<W>) e.getWindows();
+      for (W w : windows) {
+        Instant timestamp = minTimestamps.get(w);
+        if (timestamp == null || timestamp.compareTo(e.getTimestamp()) > 0) {
+          minTimestamps.put(w, e.getTimestamp());
+        } else {
+          minTimestamps.put(w, timestamp);
+        }
+
+        AccumT accum = accumulators.get(w);
+        checkState((timestamp == null && accum == null) || (timestamp != null && accum != null));
+        if (accum == null) {
+          accum = combineFn.createAccumulator(key);
+          liveWindows.add(w);
+        }
+        accum = combineFn.addInput(key, accum, e.getValue());
+        accumulators.put(w, accum);
+      }
+
+      while (!liveWindows.isEmpty()
+          && liveWindows.peek().maxTimestamp().isBefore(e.getTimestamp())) {
+        closeWindow(key, liveWindows.poll(), accumulators, minTimestamps, c);
+      }
+    }
+
+    while (!liveWindows.isEmpty()) {
+      closeWindow(key, liveWindows.poll(), accumulators, minTimestamps, c);
+    }
+  }
+
+  private void closeWindow(
+      K key, W w, Map<W, AccumT> accumulators, Map<W, Instant> minTimestamps, ProcessContext c) {
+    AccumT accum = accumulators.remove(w);
+    Instant timestamp = minTimestamps.remove(w);
+    checkState(accum != null && timestamp != null);
+    c.windowingInternals().outputWindowedValue(
+        KV.of(key, combineFn.extractOutput(key, accum)), timestamp, Arrays.asList(w));
+  }
+}
diff --git a/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsDoFn.java b/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsDoFn.java
index 2fd5a5afe4..1fdae8aa71 100644
--- a/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsDoFn.java
+++ b/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsDoFn.java
@@ -70,6 +70,11 @@ public abstract class GroupAlsoByWindowsDoFn<K, InputT, OutputT, W extends Bound
       final Coder<K> keyCoder,
       final Coder<InputT> inputCoder) {
     Preconditions.checkNotNull(combineFn);
+    if (windowingStrategy.getWindowFn().isNonMerging()
+        && windowingStrategy.getTrigger().getSpec() instanceof DefaultTrigger
+        && windowingStrategy.getMode() == AccumulationMode.DISCARDING_FIRED_PANES) {
+      return new GroupAlsoByWindowsAndCombineDoFn<>(combineFn);
+    }
     return new GABWViaOutputBufferDoFn<>(windowingStrategy,
         CombiningOutputBuffer.<K, InputT, AccumT, OutputT, W>create(
             combineFn, keyCoder, inputCoder));
diff --git a/sdk/src/test/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsDoFnTest.java b/sdk/src/test/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsDoFnTest.java
index 0fc8a2dddd..4d7449db11 100644
--- a/sdk/src/test/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsDoFnTest.java
+++ b/sdk/src/test/java/com/google/cloud/dataflow/sdk/util/GroupAlsoByWindowsDoFnTest.java
@@ -166,6 +166,48 @@ public class GroupAlsoByWindowsDoFnTest {
         Matchers.contains(window(10, 30)));
   }
 
+  @Test public void testSlidingWindowsCombine() throws Exception {
+    CombineFn<Long, ?, Long> combineFn = new Sum.SumLongFn();
+    DoFnRunner<KV<String, Iterable<WindowedValue<Long>>>, KV<String, Long>, List> runner =
+        makeRunner(
+            WindowingStrategy.of(SlidingWindows.of(Duration.millis(20)).every(Duration.millis(10))),
+            combineFn.<String>asKeyedFn());
+
+    runner.startBundle();
+
+    runner.processElement(WindowedValue.valueInEmptyWindows(
+        KV.of("k", (Iterable<WindowedValue<Long>>) Arrays.asList(
+            WindowedValue.of(
+                1L,
+                new Instant(5),
+                Arrays.asList(window(-10, 10), window(0, 20))),
+            WindowedValue.of(
+                2L,
+                new Instant(15),
+                Arrays.asList(window(0, 20), window(10, 30))),
+            WindowedValue.of(
+                4L,
+                new Instant(18),
+                Arrays.asList(window(0, 20), window(10, 30)))))));
+
+    runner.finishBundle();
+
+    List<WindowedValue<KV<String, Long>>> result = runner.getReceiver(outputTag);
+
+    assertEquals(3, result.size());
+
+    assertThat(result, Matchers.contains(
+        WindowMatchers.isSingleWindowedValue(
+            KvMatcher.isKv(Matchers.equalTo("k"), Matchers.equalTo(1L)),
+            5, -10, 10),
+        WindowMatchers.isSingleWindowedValue(
+            KvMatcher.isKv(Matchers.equalTo("k"), Matchers.equalTo(7L)),
+            5, 0, 20),
+        WindowMatchers.isSingleWindowedValue(
+            KvMatcher.isKv(Matchers.equalTo("k"), Matchers.equalTo(6L)),
+            15, 10, 30)));
+  }
+
   @Test public void testDiscontiguousWindows() throws Exception {
     DoFnRunner<KV<String, Iterable<WindowedValue<String>>>,
         KV<String, Iterable<String>>, List> runner =