Serena Williams Injured, Wimbledon Doubles Run With Venus In Doubt

Serena Williams' on-court reunion with Venus is at risk ... as the tennis superstar tweaked her right knee during her Wimbledon singles match on Tuesday.

The 44-year-old suffered the injury in the first set of her opening-round loss to Maya Joint at the All England Club, her agent Jill Smoller announced ... adding that's why Williams didn't speak with reporters after the match.

"Serena tweaked her right knee at the end of the first set and was therefore excused from her media obligations by the Wimbledon and WTA medical teams," Smoller said.

Williams is currently slated to team up with Venus for doubles on Friday ... but as of right now, it sounds like it's far from a guarantee she will be healed in time.

"She left site that night unaided and is doing everything she can to be ready for her doubles match later this week," Smoller continued.

Williams was reportedly advised not to continue playing after the ailment ... but she pushed forward. She was also offered crutches following the contest, which she declined.

It was Williams' first singles match in four years -- her comeback started at the HSBC Championship at the Queen's Club in London last month, where she competed alongside her doubles partner, 19-year-old Victoria Mboko.

Their run was cut short due to a knee injury as well -- this time, to Mboko.

Williams was able to snag the second set via tiebreaker in Tuesday's match against Joint ... but lost the first and third sets 6-3.