diff --git a/tests/phpunit/includes/bootstrap.php b/tests/phpunit/includes/bootstrap.php
index 41f8ea6a2c..460d5fbeba 100644
--- a/tests/phpunit/includes/bootstrap.php
+++ b/tests/phpunit/includes/bootstrap.php
@@ -36,6 +36,7 @@ tests_reset__SERVER();
 
 define( 'WP_TESTS_TABLE_PREFIX', $table_prefix );
 define( 'DIR_TESTDATA', dirname( __FILE__ ) . '/../data' );
+define( 'DIR_TESTROOT', realpath( dirname( dirname( __FILE__ ) ) ) );
 
 define( 'WP_LANG_DIR', DIR_TESTDATA . '/languages' );
 
diff --git a/tests/phpunit/tests/rest-api/rest-schema-setup.php b/tests/phpunit/tests/rest-api/rest-schema-setup.php
index 2159cde0d4..e5b3460122 100644
--- a/tests/phpunit/tests/rest-api/rest-schema-setup.php
+++ b/tests/phpunit/tests/rest-api/rest-schema-setup.php
@@ -395,7 +395,7 @@ class WP_Test_REST_Schema_Initialization extends WP_Test_REST_TestCase {
 		// Only generate API client fixtures in single site and when required JSON_* constants are supported.
 		if ( ! is_multisite() && version_compare( PHP_VERSION, '5.4', '>=' ) ) {
 			// Save the route object for QUnit tests.
-			$file = './tests/qunit/fixtures/wp-api-generated.js';
+			$file = dirname( DIR_TESTROOT ) . '/qunit/fixtures/wp-api-generated.js';
 			file_put_contents( $file, $mocked_responses );
 		}