Huge bummer for Serena Williams -- the tennis superstar will NOT be competing at the U.S. Open next week ... saying it's best she lets her hamstring injury fully heal before returning to the court.
The 39-year-old made the news official on her Instagram Wednesday morning ... saying she thought long and hard about it, but the experts have advised her to sit out the Grand Slam tournament in Flushing Meadows.
Remember, Serena went down with the injury in the first round of Wimbledon back in June .