Serena Williams is officially OUT of the French Open ... the tennis legend withdrew from the tournament Wednesday, saying she has an Achilles injury that's hurting like hell.
"Struggling to walk," Serena said of the injury. "So that's kind of a tell-tale sign that I should try to recover."
The 39-year-old hurt her lower leg at the U.S. Open in New York earlier this month ... and thought she had given it enough time to heal to be effective in Paris.
But, Williams said during her opening-round victory over Kristie Ahn on Monday, she felt the tendon aching -- and decided to pull out of the event.
"I had to focus on walking straight so I wasn't limping," Serena said. "And, I tried."
The good news ... Serena says she doesn't think the injury will be a long-term thing -- she told reporters she believes 4-to-6 weeks of rest and recovery should do the trick.
"I feel like my body is actually doing really, really well, and I just ran into, for lack of a better word, bad timing and bad luck, really, in New York," Williams said.
"It happened, but my body is actually doing really, really well.”
FYI -- the next major event on the tennis calendar is months away ... with the Australian Open set to begin on Jan. 18.