Serena Williams confirmed her withdrawal from the Sony Ericsson Championships in the early hours of yesterday morning, the first player to do so at the round-robin stage, and sparked confusion as to how much prize-money she is entitled to and whether she would incur a fine if she returned home to the United States.

The American, who retired after the first set of her match against Anna Chakvetadze with an injury to her left knee, was taken to hospital for a scan late on Wednesday, and it was confirmed that she had suffered inflammation behind the left patella. “I felt really sad because I heard a lot of people saying ‘Go, Serena’ and that was really nice,” Williams said. “It’s really unfortunate.”

WTA rules stipulate that all participants in the season-ending event should stay until the mandatory players’ party on Saturday and that all four semi-finalists must remain until after the final on Sunday. Last year, Maria Sharapova was fined a six-figure sum when she flew back to the US after losing in the last four, but Williams’s fine would be “peanuts” in comparison, a WTA source said.

The unfortunate departure of Williams opened the door for Marion Bartoli, the first time a reserve has been called upon since the introduction of the round-robin format in 2003. Amid much scrambling for rulebooks and only after consultation with the ATP, the WTA confirmed that Williams would be entitled to one third of Bartoli’s prize-money if the Frenchwoman finishes fourth in the yellow group. That would mean about $33,000 (about £15,600) of the $100,000 cheque that goes to the players finishing fourth. Were Bartoli to finish higher, she would keep the full amount and Williams would still receive $33,000. Bartoli also gets $20,000 for being a reserve.

Williams has suffered from quadriceps tendinitis since 2003 and had surgery on the problem that year. She played Chakvetadze yesterday with kinesio tape — a muscle-protecting treatment also used by David Beckham — running from thigh to shin on both legs. She revealed that she had been in pain since Tuesday but appeared to move comfortably until 5-4 in the first set, when she called for a medical time-out. Chakvetadze had not realised that her opponent was injured and was “surprised” when she retired after the first set.

Bartoli suffered the indignity of becoming the first player to lose 6-0, 6-0 at these championships when she came up against Justine Henin in sublime form. The world No 1 was given a standing ovation after she exacted revenge for her semi-final defeat by the Frenchwoman at Wimbledon.

Sharapova and Ana Ivanovic will advance to the semi-finals from the Red Group and they play each other in their last match. The loser has the ominous task of facing Henin.