diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index c25d543..01872ca 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -48,6 +48,7 @@ jobs:
     # Defines the WordPress and PHP Versions matrix to run tests on.
     strategy:
       fail-fast: false
+      max-parallel: 1
       matrix:
         wp-versions: [ 'latest' ] #[ '6.1.1', 'latest' ]
         php-versions: [ '8.0', '8.1', '8.2', '8.3', '8.4' ] #[ '7.4', '8.0', '8.1', '8.2' ]
diff --git a/tests/Integration/APITest.php b/tests/Integration/APITest.php
index 0ad858e..1620130 100644
--- a/tests/Integration/APITest.php
+++ b/tests/Integration/APITest.php
@@ -4245,15 +4245,15 @@ public function testUpdateSubscriberWithInvalidSubscriberID()
 	 */
 	public function testUnsubscribeByEmail()
 	{
-		// Avoid a rate limit due to previous tests.
-		sleep(2);
-
 		// Add a subscriber.
 		$emailAddress = $this->generateEmailAddress();
 		$result       = $this->api->create_subscriber($emailAddress);
 		$this->assertNotInstanceOf(\WP_Error::class, $result);
 		$this->assertIsArray($result);
 
+		// Wait a moment to ensure subscriber is created.
+		sleep(3);
+
 		// Unsubscribe.
 		$this->assertNull($this->api->unsubscribe_by_email($emailAddress));
 	}
@@ -4301,6 +4301,9 @@ public function testUnsubscribe()
 		$this->assertNotInstanceOf(\WP_Error::class, $result);
 		$this->assertIsArray($result);
 
+		// Wait a moment to ensure subscriber is created.
+		sleep(3);
+
 		// Unsubscribe.
 		$this->assertNull($this->api->unsubscribe($result['subscriber']['id']));
 	}