Serena Williams Pulls Out Of U.S. Open Due To Hamstring Injury

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 ... but her agent, Jill Smoller, told TMZ Sports last month Williams was hoping to be ready in time to compete for a 7th U.S. Open championship.

But, that ain't happening ... with Williams expressing her sadness in a heartfelt post.

"After careful consideration and following the advice of my doctors and medical team, I have decided to withdraw from the US Open to allow my body to heal completely from a torn hamstring," Serena said.

"New York is one of the most exciting cities in the world and one of my favorite places to play -- I'll miss seeing the fans but will be cheering everyone on from afar."

It's a massive blow to the tournament -- which was already without big names like Roger Federer and Rafael Nadal -- but here's hoping Serena can get back out there sooner than later.