diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index d119f7f..4f5e692 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -122,22 +122,22 @@ function _simpletest_format_summary_line($summary) {
  *   The test ID.
  */
 function simpletest_run_tests($test_list) {
-  $test_id = db_insert('simpletest_test_id')
-    ->useDefaults(array('test_id'))
-    ->execute();
-
-  if (!empty($test_list['phpunit'])) {
-    $phpunit_results = simpletest_run_phpunit_tests($test_id, $test_list['phpunit']);
-    simpletest_process_phpunit_results($phpunit_results);
+  // We used to separate PHPUnit and Simpletest tests for a performance
+  // optimization. In order to support backwards compatibility check if these
+  // keys are set and create a single test list.
+  // @todo https://www.drupal.org/node/2748967 Remove BC support in Drupal 9.
+  if (isset($test_list['simpletest'])) {
+    $test_list = array_merge($test_list, $test_list['simpletest']);
+    unset($test_list['simpletest']);
   }
-
-  // Early return if there are no further tests to run.
-  if (empty($test_list['simpletest'])) {
-    return $test_id;
+  if (isset($test_list['phpunit'])) {
+    $test_list = array_merge($test_list, $test_list['phpunit']);
+    unset($test_list['phpunit']);
   }
 
-  // Continue with SimpleTests only.
-  $test_list = $test_list['simpletest'];
+  $test_id = db_insert('simpletest_test_id')
+    ->useDefaults(array('test_id'))
+    ->execute();
 
   // Clear out the previous verbose files.
   file_unmanaged_delete_recursive('public://simpletest/verbose');
@@ -190,7 +190,7 @@ function simpletest_run_phpunit_tests($test_id, array $unescaped_test_classnames
     // Something broke during the execution of phpunit.
     // Return an error record of all failed classes.
     $rows[] = [
-      'test_id' => '1',
+      'test_id' => $test_id,
       'test_class' => implode(",", $unescaped_test_classnames),
       'status' => 'fail',
       'message' => 'PHPunit Test failed to complete',
@@ -224,6 +224,48 @@ function simpletest_process_phpunit_results($phpunit_results) {
 }
 
 /**
+ * Maps phpunit results to a data structure for batch messages and run-tests.sh.
+ *
+ * @param array $results
+ *   The output from simpletest_run_phpunit_tests().
+ *
+ * @return array
+ *   The test result summary. A row per test class.
+ */
+function simpletest_summarise_phpunit_result($results) {
+  $summaries = [];
+  foreach ($results as $result) {
+    if (!isset($summaries[$result['test_class']])) {
+      $summaries[$result['test_class']] = array(
+        '#pass' => 0,
+        '#fail' => 0,
+        '#exception' => 0,
+        '#debug' => 0,
+      );
+    }
+
+    switch ($result['status']) {
+      case 'pass':
+        $summaries[$result['test_class']]['#pass']++;
+        break;
+
+      case 'fail':
+        $summaries[$result['test_class']]['#fail']++;
+        break;
+
+      case 'exception':
+        $summaries[$result['test_class']]['#exception']++;
+        break;
+
+      case 'debug':
+        $summaries[$result['test_class']]['#debug']++;
+        break;
+    }
+  }
+  return $summaries;
+}
+
+/**
  * Returns the path to use for PHPUnit's --log-junit option.
  *
  * @param $test_id
@@ -352,15 +394,21 @@ function _simpletest_batch_operation($test_list_init, $test_id, &$context) {
 
   // Perform the next test.
   $test_class = array_shift($test_list);
-  $test = new $test_class($test_id);
-  $test->run();
+  if (is_subclass_of($test_class, \PHPUnit_Framework_TestCase::class)) {
+    $phpunit_results = simpletest_run_phpunit_tests($test_id, [$test_class]);
+    simpletest_process_phpunit_results($phpunit_results);
+    $test_results[$test_class] = simpletest_summarise_phpunit_result($phpunit_results)[$test_class];
+  }
+  else {
+    $test = new $test_class($test_id);
+    $test->run();
+    \Drupal::moduleHandler()->invokeAll('test_finished', array($test->results));
+    $test_results[$test_class] = $test->results;
+  }
   $size = count($test_list);
   $info = TestDiscovery::getTestInfo($test_class);
 
-  \Drupal::moduleHandler()->invokeAll('test_finished', array($test->results));
-
   // Gather results and compose the report.
-  $test_results[$test_class] = $test->results;
   foreach ($test_results[$test_class] as $key => $value) {
     $test_results[$key] += $value;
   }
diff --git a/core/modules/simpletest/src/Form/SimpletestTestForm.php b/core/modules/simpletest/src/Form/SimpletestTestForm.php
index 1c58efe..e645705 100644
--- a/core/modules/simpletest/src/Form/SimpletestTestForm.php
+++ b/core/modules/simpletest/src/Form/SimpletestTestForm.php
@@ -217,18 +217,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       $form_state->setValue('tests', $user_input['tests']);
     }
 
-    $tests_list = array();
-    foreach ($form_state->getValue('tests') as $class_name => $value) {
-      if ($value === $class_name) {
-        if (is_subclass_of($class_name, 'PHPUnit_Framework_TestCase')) {
-          $test_type = 'phpunit';
-        }
-        else {
-          $test_type = 'simpletest';
-        }
-        $tests_list[$test_type][] = $class_name;
-      }
-    }
+    $tests_list = array_filter($form_state->getValue('tests'));
     if (!empty($tests_list)) {
       putenv('SIMPLETEST_BASE_URL=' . $base_url);
       $test_id = simpletest_run_tests($tests_list, 'drupal');
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index df61328..c0afeb9 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -690,36 +690,7 @@ function simpletest_script_run_phpunit($test_id, $class) {
 
   // Map phpunit results to a data structure we can pass to
   // _simpletest_format_summary_line.
-  $summaries = array();
-  foreach ($results as $result) {
-    if (!isset($summaries[$result['test_class']])) {
-      $summaries[$result['test_class']] = array(
-        '#pass' => 0,
-        '#fail' => 0,
-        '#exception' => 0,
-        '#debug' => 0,
-      );
-    }
-
-    switch ($result['status']) {
-      case 'pass':
-        $summaries[$result['test_class']]['#pass']++;
-        break;
-
-      case 'fail':
-        $summaries[$result['test_class']]['#fail']++;
-        break;
-
-      case 'exception':
-        $summaries[$result['test_class']]['#exception']++;
-        break;
-
-      case 'debug':
-        $summaries[$result['test_class']]['#debug']++;
-        break;
-    }
-  }
-
+  $summaries = simpletest_summarise_phpunit_result($results);
   foreach ($summaries as $class => $summary) {
     simpletest_script_reporter_display_summary($class, $summary);
   }