diff --git a/libcxx/include/__threading_support b/libcxx/include/__threading_support
index 46ac7d8b362c..0d1f1e681f50 100644
--- a/libcxx/include/__threading_support
+++ b/libcxx/include/__threading_support
@@ -79,7 +79,7 @@ typedef pthread_t __libcpp_thread_t;
 typedef pthread_key_t __libcpp_tls_key;
 
 #define _LIBCPP_TLS_DESTRUCTOR_CC
-#else
+#elif !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 // Mutex
 typedef void* __libcpp_mutex_t;
 #define _LIBCPP_MUTEX_INITIALIZER 0
@@ -112,8 +112,9 @@ typedef void* __libcpp_thread_t;
 typedef long __libcpp_tls_key;
 
 #define _LIBCPP_TLS_DESTRUCTOR_CC __stdcall
-#endif // defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#endif // !defined(_LIBCPP_HAS_THREAD_API_PTHREAD) && !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 
+#if !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 // Mutex
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_recursive_mutex_init(__libcpp_recursive_mutex_t *__m);
@@ -208,6 +209,8 @@ void *__libcpp_tls_get(__libcpp_tls_key __key);
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_tls_set(__libcpp_tls_key __key, void *__p);
 
+#endif // !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
+
 #if (!defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
      defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL)) && \
     defined(_LIBCPP_HAS_THREAD_API_PTHREAD)