Williams has had an injury-plagued year

Williams was losing 5-4 in the first set when she was treated on her heavily strapped left knee by the doctor.

"It's beyond unfortunate and I can't stress how disappointed I am," said Williams, who retired at 6-4.

It means Justine Henin, who earlier beat Jelena Jankovic 6-2 6-2, is definitely through to the semi-finals.

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Williams was taken to hospital for a scan and will not make a statement about her presence in the tournament until the results are known.

Henin, who went into the match with a commanding 8-0 career record against the 22-year-old Serb, soon found her range for a 2-0 lead.

Jankovic fought back and despite losing three break points won the fourth game to level at 2-2, but Henin won the next four to take the set in 35 minutes.

As Jankovic's frustration grew, Henin sealed victory - her second of the tournament.

When Jankovic served a double fault at the start of the second set it seemed as though she would quickly subside.

Her persistence was admirable but she quickly became frustrated by her inability to overcome Henin's masterful control of the court.

Henin maintained her focus against an out of sorts Jankovic

After two double faults in the final game of the opening set, Henin served two more in her opening service game of the next.

That helped to create a break point for Jankovic but a stunning cross-court backhand got Henin out of danger, and after securing a gruelling second game she dictated the remainder of the contest.

"It's important to play every point 100%," Henin said. "I need to keep working on my serve but generally I can be very happy."

In the other group, Serbia's Ana Ivanovic moved a step closer to the last four by beating Slovak Daniela Hantuchova 6-2 7-6 (11-9).