LONDON (AP) — Serena Williams’ doubles campaign at Queen’s Club could be over after one match.

Victoria Mboko, the American star’s doubles partner, retired from her singles match against Karolina Pliskova on Wednesday after sustaining a knee injury in the second set.

Williams and Mboko had won their opening doubles match Tuesday — in the 44-year-old Williams’ return to professional tennis after nearly four years away from the sport — and are scheduled to play again Thursday.

As of Wednesday night, Mboko and Williams remained on Thursday’s schedule to face Leylah Fernandez and Laura Siegemund in the quarterfinals. It’s set to be the last match of the day at the HSBC Championships.

While trying to return a shot against Pliskova, the 19-year-old Canadian slipped behind the baseline and immediately grabbed her left knee. Mboko had lost the first set 6-2 and was up 4-3 in the second against the 2021 Wimbledon runner-up, who was facing a break point.

Mboko got up on her own but then told a physiotherapist there was “no stability right now.”

She walked gingerly to her bench and put a towel to her face as she sat down. Minutes later, she retired from the match.

Williams had previously announced that she will also play doubles at the Berlin Open next week.

On Tuesday, Williams and Mboko beat third-seeded duo Nicole Melichar-Martinez and Erin Routliffe 7-6 (2), 6-2 in Williams’ first professional match since the 2022 U.S. Open.

Mboko is ranked No. 9 in singles.

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AP tennis: https://apnews.com/hub/tennis