diff --git a/tsdb/chunks/chunks.go b/tsdb/chunks/chunks.go
index 1e78351db..acdcc4a17 100644
--- a/tsdb/chunks/chunks.go
+++ b/tsdb/chunks/chunks.go
@@ -601,14 +601,14 @@ func (s *Reader) Chunk(ref uint64) (chunkenc.Chunk, error) {
 }
 
 func nextSequenceFile(dir string) (string, int, error) {
-	names, err := fileutil.ReadDir(dir)
+	files, err := ioutil.ReadDir(dir)
 	if err != nil {
 		return "", 0, err
 	}
 
 	i := uint64(0)
-	for _, n := range names {
-		j, err := strconv.ParseUint(n, 10, 64)
+	for _, f := range files {
+		j, err := strconv.ParseUint(f.Name(), 10, 64)
 		if err != nil {
 			continue
 		}
diff --git a/tsdb/db.go b/tsdb/db.go
index 2a78a0b1a..97df4b3c4 100644
--- a/tsdb/db.go
+++ b/tsdb/db.go
@@ -1427,14 +1427,14 @@ func sequenceFiles(dir string) ([]string, error) {
 }
 
 func nextSequenceFile(dir string) (string, int, error) {
-	names, err := fileutil.ReadDir(dir)
+	files, err := ioutil.ReadDir(dir)
 	if err != nil {
 		return "", 0, err
 	}
 
 	i := uint64(0)
-	for _, n := range names {
-		j, err := strconv.ParseUint(n, 10, 64)
+	for _, f := range files {
+		j, err := strconv.ParseUint(f.Name(), 10, 64)
 		if err != nil {
 			continue
 		}
diff --git a/tsdb/fileutil/fileutil.go b/tsdb/fileutil/fileutil.go
index 4088f522a..be9c6f226 100644
--- a/tsdb/fileutil/fileutil.go
+++ b/tsdb/fileutil/fileutil.go
@@ -21,7 +21,6 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	"sort"
 	"strings"
 )
 
@@ -91,21 +90,6 @@ func readDirs(src string) ([]string, error) {
 	return files, nil
 }
 
-// ReadDir returns the filenames in the given directory in sorted order.
-func ReadDir(dirpath string) ([]string, error) {
-	dir, err := os.Open(dirpath)
-	if err != nil {
-		return nil, err
-	}
-	defer dir.Close()
-	names, err := dir.Readdirnames(-1)
-	if err != nil {
-		return nil, err
-	}
-	sort.Strings(names)
-	return names, nil
-}
-
 // Rename safely renames a file.
 func Rename(from, to string) error {
 	if err := os.Rename(from, to); err != nil {
diff --git a/tsdb/wal/checkpoint_test.go b/tsdb/wal/checkpoint_test.go
index eb0e22a4b..3930b548b 100644
--- a/tsdb/wal/checkpoint_test.go
+++ b/tsdb/wal/checkpoint_test.go
@@ -24,7 +24,6 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/prometheus/prometheus/pkg/labels"
-	"github.com/prometheus/prometheus/tsdb/fileutil"
 	"github.com/prometheus/prometheus/tsdb/record"
 	"github.com/prometheus/prometheus/util/testutil"
 )
@@ -92,9 +91,13 @@ func TestDeleteCheckpoints(t *testing.T) {
 
 	testutil.Ok(t, DeleteCheckpoints(dir, 2))
 
-	files, err := fileutil.ReadDir(dir)
+	files, err := ioutil.ReadDir(dir)
 	testutil.Ok(t, err)
-	testutil.Equals(t, []string{"checkpoint.02", "checkpoint.03"}, files)
+	fns := []string{}
+	for _, f := range files {
+		fns = append(fns, f.Name())
+	}
+	testutil.Equals(t, []string{"checkpoint.02", "checkpoint.03"}, fns)
 
 	testutil.Ok(t, os.MkdirAll(filepath.Join(dir, "checkpoint.99999999"), 0777))
 	testutil.Ok(t, os.MkdirAll(filepath.Join(dir, "checkpoint.100000000"), 0777))
@@ -102,9 +105,13 @@ func TestDeleteCheckpoints(t *testing.T) {
 
 	testutil.Ok(t, DeleteCheckpoints(dir, 100000000))
 
-	files, err = fileutil.ReadDir(dir)
+	files, err = ioutil.ReadDir(dir)
 	testutil.Ok(t, err)
-	testutil.Equals(t, []string{"checkpoint.100000000", "checkpoint.100000001"}, files)
+	fns = []string{}
+	for _, f := range files {
+		fns = append(fns, f.Name())
+	}
+	testutil.Equals(t, []string{"checkpoint.100000000", "checkpoint.100000001"}, fns)
 }
 
 func TestCheckpoint(t *testing.T) {
@@ -178,10 +185,10 @@ func TestCheckpoint(t *testing.T) {
 			testutil.Ok(t, DeleteCheckpoints(w.Dir(), 106))
 
 			// Only the new checkpoint should be left.
-			files, err := fileutil.ReadDir(dir)
+			files, err := ioutil.ReadDir(dir)
 			testutil.Ok(t, err)
 			testutil.Equals(t, 1, len(files))
-			testutil.Equals(t, "checkpoint.00000106", files[0])
+			testutil.Equals(t, "checkpoint.00000106", files[0].Name())
 
 			sr, err := NewSegmentsReader(filepath.Join(dir, "checkpoint.00000106"))
 			testutil.Ok(t, err)
diff --git a/tsdb/wal/wal.go b/tsdb/wal/wal.go
index 30d8b002e..e5f58c034 100644
--- a/tsdb/wal/wal.go
+++ b/tsdb/wal/wal.go
@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"hash/crc32"
 	"io"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sort"
@@ -764,11 +765,12 @@ type segmentRef struct {
 }
 
 func listSegments(dir string) (refs []segmentRef, err error) {
-	files, err := fileutil.ReadDir(dir)
+	files, err := ioutil.ReadDir(dir)
 	if err != nil {
 		return nil, err
 	}
-	for _, fn := range files {
+	for _, f := range files {
+		fn := f.Name()
 		k, err := strconv.Atoi(fn)
 		if err != nil {
 			continue
diff --git a/tsdb/wal/watcher.go b/tsdb/wal/watcher.go
index 4a9e7f455..7e1a89145 100644
--- a/tsdb/wal/watcher.go
+++ b/tsdb/wal/watcher.go
@@ -16,6 +16,7 @@ package wal
 import (
 	"fmt"
 	"io"
+	"io/ioutil"
 	"math"
 	"os"
 	"path"
@@ -29,7 +30,6 @@ import (
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/prometheus/pkg/timestamp"
-	"github.com/prometheus/prometheus/tsdb/fileutil"
 	"github.com/prometheus/prometheus/tsdb/record"
 )
 
@@ -293,15 +293,15 @@ func (w *Watcher) firstAndLast() (int, int, error) {
 // Copied from tsdb/wal/wal.go so we do not have to open a WAL.
 // Plan is to move WAL watcher to TSDB and dedupe these implementations.
 func (w *Watcher) segments(dir string) ([]int, error) {
-	files, err := fileutil.ReadDir(dir)
+	files, err := ioutil.ReadDir(dir)
 	if err != nil {
 		return nil, err
 	}
 
 	var refs []int
 	var last int
-	for _, fn := range files {
-		k, err := strconv.Atoi(fn)
+	for _, f := range files {
+		k, err := strconv.Atoi(f.Name())
 		if err != nil {
 			continue
 		}
diff --git a/tsdb/wal_test.go b/tsdb/wal_test.go
index cdb23adaf..b5346d091 100644
--- a/tsdb/wal_test.go
+++ b/tsdb/wal_test.go
@@ -28,7 +28,6 @@ import (
 
 	"github.com/go-kit/kit/log"
 	"github.com/prometheus/prometheus/pkg/labels"
-	"github.com/prometheus/prometheus/tsdb/fileutil"
 	"github.com/prometheus/prometheus/tsdb/record"
 	"github.com/prometheus/prometheus/tsdb/tombstones"
 	"github.com/prometheus/prometheus/tsdb/wal"
@@ -303,8 +302,12 @@ func TestWALRestoreCorrupted_invalidSegment(t *testing.T) {
 	_, err = OpenSegmentWAL(dir, log.NewLogfmtLogger(os.Stderr), 0, nil)
 	testutil.Ok(t, err)
 
-	fns, err := fileutil.ReadDir(dir)
+	files, err := ioutil.ReadDir(dir)
 	testutil.Ok(t, err)
+	fns := []string{}
+	for _, f := range files {
+		fns = append(fns, f.Name())
+	}
 	testutil.Equals(t, []string{"000000"}, fns)
 }