diff --git a/test/common/tmpdir.js b/test/common/tmpdir.js
index 33b2264a8d69f5..92bcc9521a8bdb 100644
--- a/test/common/tmpdir.js
+++ b/test/common/tmpdir.js
@@ -89,7 +89,8 @@ const testRoot = process.env.NODE_TEST_DIR ?
 
 // Using a `.` prefixed name, which is the convention for "hidden" on POSIX,
 // gets tools to ignore it by default or by simple rules, especially eslint.
-const tmpdirName = '.tmp.' + (process.env.TEST_THREAD_ID || '0');
+const tmpdirName = '.tmp.' +
+  (process.env.TEST_SERIAL_ID || process.env.TEST_THREAD_ID || '0');
 const tmpPath = path.join(testRoot, tmpdirName);
 
 function refresh(opts = {}) {
diff --git a/tools/test.py b/tools/test.py
index ff08749425eb5a..0726619bd092a0 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -77,6 +77,7 @@ class ProgressIndicator(object):
 
   def __init__(self, cases, flaky_tests_mode):
     self.cases = cases
+    self.serial_id = 0
     self.flaky_tests_mode = flaky_tests_mode
     self.parallel_queue = Queue(len(cases))
     self.sequential_queue = Queue(len(cases))
@@ -146,6 +147,8 @@ def RunSingle(self, parallel, thread_id):
       case = test
       case.thread_id = thread_id
       self.lock.acquire()
+      case.serial_id = self.serial_id
+      self.serial_id += 1
       self.AboutToRun(case)
       self.lock.release()
       try:
@@ -504,6 +507,7 @@ def __init__(self, context, path, arch, mode):
     self.mode = mode
     self.parallel = False
     self.disable_core_files = False
+    self.serial_id = 0
     self.thread_id = 0
 
   def IsNegative(self):
@@ -535,6 +539,7 @@ def RunCommand(self, command, env):
   def Run(self):
     try:
       result = self.RunCommand(self.GetCommand(), {
+        "TEST_SERIAL_ID": "%d" % self.serial_id,
         "TEST_THREAD_ID": "%d" % self.thread_id,
         "TEST_PARALLEL" : "%d" % self.parallel
       })