diff --git a/lucene/classification/src/java/org/apache/lucene/classification/KNearestNeighborClassifier.java b/lucene/classification/src/java/org/apache/lucene/classification/KNearestNeighborClassifier.java
index 77f04164cc3..fdcef140921 100644
--- a/lucene/classification/src/java/org/apache/lucene/classification/KNearestNeighborClassifier.java
+++ b/lucene/classification/src/java/org/apache/lucene/classification/KNearestNeighborClassifier.java
@@ -38,6 +38,7 @@ import org.apache.lucene.search.Query;
 import org.apache.lucene.search.ScoreDoc;
 import org.apache.lucene.search.TopDocs;
 import org.apache.lucene.search.WildcardQuery;
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.ClassicSimilarity;
 import org.apache.lucene.search.similarities.Similarity;
 import org.apache.lucene.util.BytesRef;
@@ -106,7 +107,7 @@ public class KNearestNeighborClassifier implements Classifier<BytesRef> {
     if (similarity != null) {
       this.indexSearcher.setSimilarity(similarity);
     } else {
-      this.indexSearcher.setSimilarity(new ClassicSimilarity());
+      this.indexSearcher.setSimilarity(new BM25Similarity());
     }
     if (minDocsFreq > 0) {
       mlt.setMinDocFreq(minDocsFreq);
@@ -124,7 +125,10 @@ public class KNearestNeighborClassifier implements Classifier<BytesRef> {
    */
   @Override
   public ClassificationResult<BytesRef> assignClass(String text) throws IOException {
-    TopDocs knnResults = knnSearch(text);
+    return classifyFromTopDocs(knnSearch(text));
+  }
+
+  protected ClassificationResult<BytesRef> classifyFromTopDocs(TopDocs knnResults) throws IOException {
     List<ClassificationResult<BytesRef>> assignedClasses = buildListFromTopDocs(knnResults);
     ClassificationResult<BytesRef> assignedClass = null;
     double maxscore = -Double.MAX_VALUE;
diff --git a/lucene/classification/src/java/org/apache/lucene/classification/document/KNearestNeighborDocumentClassifier.java b/lucene/classification/src/java/org/apache/lucene/classification/document/KNearestNeighborDocumentClassifier.java
index e01090a9cac..adcb13b6a27 100644
--- a/lucene/classification/src/java/org/apache/lucene/classification/document/KNearestNeighborDocumentClassifier.java
+++ b/lucene/classification/src/java/org/apache/lucene/classification/document/KNearestNeighborDocumentClassifier.java
@@ -77,17 +77,7 @@ public class KNearestNeighborDocumentClassifier extends KNearestNeighborClassifi
    */
   @Override
   public ClassificationResult<BytesRef> assignClass(Document document) throws IOException {
-    TopDocs knnResults = knnSearch(document);
-    List<ClassificationResult<BytesRef>> assignedClasses = buildListFromTopDocs(knnResults);
-    ClassificationResult<BytesRef> assignedClass = null;
-    double maxscore = -Double.MAX_VALUE;
-    for (ClassificationResult<BytesRef> cl : assignedClasses) {
-      if (cl.getScore() > maxscore) {
-        assignedClass = cl;
-        maxscore = cl.getScore();
-      }
-    }
-    return assignedClass;
+    return classifyFromTopDocs(knnSearch(document));
   }
 
   /**
diff --git a/lucene/classification/src/test/org/apache/lucene/classification/document/KNearestNeighborDocumentClassifierTest.java b/lucene/classification/src/test/org/apache/lucene/classification/document/KNearestNeighborDocumentClassifierTest.java
index 8c885fb9c72..a323724e53d 100644
--- a/lucene/classification/src/test/org/apache/lucene/classification/document/KNearestNeighborDocumentClassifierTest.java
+++ b/lucene/classification/src/test/org/apache/lucene/classification/document/KNearestNeighborDocumentClassifierTest.java
@@ -33,8 +33,9 @@ public class KNearestNeighborDocumentClassifierTest extends DocumentClassificati
     try {
       Document videoGameDocument = getVideoGameDocument();
       Document batmanDocument = getBatmanDocument();
-      checkCorrectDocumentClassification(new KNearestNeighborDocumentClassifier(indexReader,null, null, 1, 1, 1, categoryFieldName, field2analyzer, new String[]{textFieldName, titleFieldName, authorFieldName}), videoGameDocument, VIDEOGAME_RESULT);
-      checkCorrectDocumentClassification(new KNearestNeighborDocumentClassifier(indexReader,null, null, 1, 1, 1, categoryFieldName, field2analyzer, new String[]{textFieldName, titleFieldName, authorFieldName}), batmanDocument, BATMAN_RESULT);
+      KNearestNeighborDocumentClassifier classifier = new KNearestNeighborDocumentClassifier(indexReader, null, null, 1, 4, 1, categoryFieldName, field2analyzer, new String[]{textFieldName, titleFieldName, authorFieldName});
+      checkCorrectDocumentClassification(classifier, videoGameDocument, VIDEOGAME_RESULT);
+      checkCorrectDocumentClassification(classifier, batmanDocument, BATMAN_RESULT);
       // considering only the text we have wrong classification because the text was ambiguos on purpose
       checkCorrectDocumentClassification(new KNearestNeighborDocumentClassifier(indexReader,null, null, 1, 1, 1, categoryFieldName, field2analyzer, new String[]{textFieldName}), videoGameDocument, BATMAN_RESULT);
       checkCorrectDocumentClassification(new KNearestNeighborDocumentClassifier(indexReader,null, null, 1, 1, 1, categoryFieldName, field2analyzer, new String[]{textFieldName}), batmanDocument, VIDEOGAME_RESULT);
@@ -51,9 +52,10 @@ public class KNearestNeighborDocumentClassifierTest extends DocumentClassificati
     try {
       Document videoGameDocument = getVideoGameDocument();
       Document batmanDocument = getBatmanDocument();
-      double score1 = checkCorrectDocumentClassification(new KNearestNeighborDocumentClassifier(indexReader,null, null, 1, 1, 1, categoryFieldName, field2analyzer, new String[]{textFieldName, titleFieldName, authorFieldName}), videoGameDocument, VIDEOGAME_RESULT);
+      KNearestNeighborDocumentClassifier classifier = new KNearestNeighborDocumentClassifier(indexReader, null, null, 1, 4, 1, categoryFieldName, field2analyzer, new String[]{textFieldName, titleFieldName, authorFieldName});
+      double score1 = checkCorrectDocumentClassification(classifier, videoGameDocument, VIDEOGAME_RESULT);
       assertEquals(1.0,score1,0);
-      double score2 = checkCorrectDocumentClassification(new KNearestNeighborDocumentClassifier(indexReader,null, null, 1, 1, 1, categoryFieldName, field2analyzer, new String[]{textFieldName, titleFieldName, authorFieldName}), batmanDocument, BATMAN_RESULT);
+      double score2 = checkCorrectDocumentClassification(classifier, batmanDocument, BATMAN_RESULT);
       assertEquals(1.0,score2,0);
       // considering only the text we have wrong classification because the text was ambiguos on purpose
       double score3 = checkCorrectDocumentClassification(new KNearestNeighborDocumentClassifier(indexReader,null, null, 1, 1, 1, categoryFieldName, field2analyzer, new String[]{textFieldName}), videoGameDocument, BATMAN_RESULT);