A worried Venus Williams looks on during a break in her semi-final against Kim Clijsters in Los Angeles Sunday. Williams left the match due to an unspecified injury after losing the first five games. (Reuters)

Los Angeles: World No. 1 Serena Williams, seeking to cap a triumphant year with a second straight title in the WTA Championship, rallied from a set down to subdue third seed Jennifer Capriati in the semi-finals Sunday.

But the now-familiar all-Williams final failed to materialise as world No. 2 Venus Williams, runner-up to her younger sister in three Grand Slams this season, retired with a lower leg strain while trailing Kim Clijsters 5-0 in the first set.

“I just hope she’s doing ok,” Serena said of Venus. “I was upset that she actually went out on court because she was injured.”

Serena rallied from a set down to defeat Capriati 2-6, 6-4, 6-4. She posted her seventh winover Capriati in 11 meetings, and has now won their last six meetings.Capriati put the pressure on early, making just four unforced errors in the opening set to Serena’s 14.

But as the match progressed, Serena’s ability to hit winners and come up with big serves when she needed them began to override Capriati’s ability to cover the court and extend the rallies.

Serena grabbed the early break in the second set, gave it back in the seventh game then broke again in the 10th to get back on even terms.

Capriati, leading 3-1 in the third set thanks to a service break in the third game, was one point away from taking a 4-1 lead. But Serena saved the break point with a service winner and went on to hold and narrow the gap to 3-2.

Serena then broke Capriati to level the set and broke her again in the final game to capture the match on her first match point.

“Why, why, why? I don’t know, that’s just the way a tennis match is,” Capriati said of her inability to make more of her chances in the third. “It’s pretty good that I get the break points. I think she played tough on those points. It’s just a matter of a few points here and there.”

Serena was equally unable to pinpoint a precise reason for her victory. “I don’t know, she just hit a ball out at the end, and that was it,” she said.

Although Serena was disappointed with her number of errors in each of her last two matches here, she takes an 18-match winning streak into the final.

She hasn’t lost since a quarter final dismissal by Chanda Rubin at Los Angeles in April.

By winning this event, Serena will become the first woman to earn more than $ 4 million in a season and become the first woman since Steffi Graf in 1995-96 to successfully defend her WTA Championship title.

She has won all five of her matches against Clijsters.

The fifth-seeded Belgian led 5-0 when Venus, after consulting the trainer, called it a day. She had started the match with her left calf and ankle taped.