diff --git a/rules/ast/ast.go b/rules/ast/ast.go
index a6f9e0195a..a3eb257888 100644
--- a/rules/ast/ast.go
+++ b/rules/ast/ast.go
@@ -19,7 +19,6 @@ import (
 	"fmt"
 	"hash/fnv"
 	"math"
-	"sort"
 	"time"
 
 	clientmodel "github.com/prometheus/client_golang/model"
@@ -375,28 +374,8 @@ func (node *ScalarFunctionCall) Eval(timestamp clientmodel.Timestamp) clientmode
 func (node *VectorAggregation) labelsToGroupingKey(labels clientmodel.Metric) uint64 {
 	summer := fnv.New64a()
 	for _, label := range node.groupBy {
-		fmt.Fprint(summer, labels[label])
-	}
-
-	return summer.Sum64()
-}
-
-func labelsToKey(labels clientmodel.Metric) uint64 {
-	pairs := metric.LabelPairs{}
-
-	for label, value := range labels {
-		pairs = append(pairs, &metric.LabelPair{
-			Name:  label,
-			Value: value,
-		})
-	}
-
-	sort.Sort(pairs)
-
-	summer := fnv.New64a()
-
-	for _, pair := range pairs {
-		fmt.Fprint(summer, pair.Name, pair.Value)
+		summer.Write([]byte(labels[label]))
+		summer.Write([]byte{clientmodel.SeparatorByte})
 	}
 
 	return summer.Sum64()
@@ -435,7 +414,7 @@ func EvalVectorRange(node VectorNode, start clientmodel.Timestamp, end clientmod
 	defer closer.Close()
 
 	evalTimer := queryStats.GetTimer(stats.InnerEvalTime).Start()
-	sampleStreams := map[uint64]*SampleStream{}
+	sampleStreams := map[clientmodel.Fingerprint]*SampleStream{}
 	for t := start; !t.After(end); t = t.Add(interval) {
 		if et := totalEvalTimer.ElapsedTime(); et > *queryTimeout {
 			evalTimer.Stop()
@@ -447,14 +426,14 @@ func EvalVectorRange(node VectorNode, start clientmodel.Timestamp, end clientmod
 				Value:     sample.Value,
 				Timestamp: sample.Timestamp,
 			}
-			groupingKey := labelsToKey(sample.Metric.Metric)
-			if sampleStreams[groupingKey] == nil {
-				sampleStreams[groupingKey] = &SampleStream{
+			fp := sample.Metric.Metric.Fingerprint()
+			if sampleStreams[fp] == nil {
+				sampleStreams[fp] = &SampleStream{
 					Metric: sample.Metric,
 					Values: metric.Values{samplePair},
 				}
 			} else {
-				sampleStreams[groupingKey].Values = append(sampleStreams[groupingKey].Values, samplePair)
+				sampleStreams[fp].Values = append(sampleStreams[fp].Values, samplePair)
 			}
 		}
 	}
diff --git a/rules/helpers_test.go b/rules/helpers_test.go
index ab1da04e89..867f668918 100644
--- a/rules/helpers_test.go
+++ b/rules/helpers_test.go
@@ -184,6 +184,27 @@ var testMatrix = ast.Matrix{
 		},
 		Values: append(getTestValueStream(0, 90, 10, testStartTime), getTestValueStream(0, 0, 10, testStartTime.Add(testSampleInterval*10))...),
 	},
+	// For label-key grouping regression test.
+	{
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "label_grouping_test",
+				"a": "aa",
+				"b": "bb",
+			},
+		},
+		Values: getTestValueStream(0, 100, 10, testStartTime),
+	},
+	{
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "label_grouping_test",
+				"a": "a",
+				"b": "abb",
+			},
+		},
+		Values: getTestValueStream(0, 200, 20, testStartTime),
+	},
 }
 
 var testVector = getTestVectorFromTestMatrix(testMatrix)
diff --git a/rules/rules_test.go b/rules/rules_test.go
index 2661e24ab4..d25cd25012 100644
--- a/rules/rules_test.go
+++ b/rules/rules_test.go
@@ -553,6 +553,8 @@ func TestExpressions(t *testing.T) {
 				`testcounter_reset_end => 0 @[%v]`,
 				`testcounter_reset_middle => 50 @[%v]`,
 				`x{y="testvalue"} => 100 @[%v]`,
+				`label_grouping_test{a="a", b="abb"} => 200 @[%v]`,
+				`label_grouping_test{a="aa", b="bb"} => 100 @[%v]`,
 			},
 		},
 		{
@@ -641,6 +643,14 @@ func TestExpressions(t *testing.T) {
 			expr:       `sum(http_requests) offset 5m`,
 			shouldFail: true,
 		},
+		// Regression test for missing separator byte in labelsToGroupingKey.
+		{
+			expr: `sum(label_grouping_test) by (a, b)`,
+			output: []string{
+				`{a="a", b="abb"} => 200 @[%v]`,
+				`{a="aa", b="bb"} => 100 @[%v]`,
+			},
+		},
 	}
 
 	storage, closer := newTestStorage(t)
@@ -850,6 +860,70 @@ func TestRangedEvaluationRegressions(t *testing.T) {
 			},
 			expr: "sum(testmetric) keeping_extra",
 		},
+		{
+			// Testing metric fingerprint grouping behavior.
+			in: ast.Matrix{
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"aa": "bb",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     1,
+						},
+					},
+				},
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"a": "abb",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     2,
+						},
+					},
+				},
+			},
+			out: ast.Matrix{
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"aa": "bb",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     1,
+						},
+					},
+				},
+				{
+					Metric: clientmodel.COWMetric{
+						Metric: clientmodel.Metric{
+							clientmodel.MetricNameLabel: "testmetric",
+							"a": "abb",
+						},
+					},
+					Values: metric.Values{
+						{
+							Timestamp: testStartTime,
+							Value:     2,
+						},
+					},
+				},
+			},
+			expr: "testmetric",
+		},
 	}
 
 	for i, s := range scenarios {