Victoria Azarenka was four points from the finish line — a coveted place in the Sony Ericsson Championships semifinals — as she opened a commanding 6-4, 5-2 lead over alternate Agnieszka Radwanska in today's final round-robin match at the Khalifa Tennis Complex in Doha.

But rather than creating closure to sew up her semifinal spot, Azarenka wilted under the pressure in surrendering six straight games then succumbed to a left leg injury in staggering out of the event. Radwanska, who tormented the limping Belarusian with a series of well-placed drop shots, held a 4-6, 7-5, 4-1 lead when Azarenka retired.

"I didn't expect I would play this week. I'm very, very happy I would have a chance and I was fighting to the end," said Radwanska, who replaced fellow alternate Vera Zvonareva in the field after Zvonareva pulled out. "I knew this was my last match and I kept fighting to the end. That's tennis — you have to keep going up to the last point and I'm very happy I did it."

The retirement means Caroline Wozniacki, who posted a 2-1 record in White Group play, joins second-ranked Serena Williams, defending Doha champion Venus Williams and 2008 year-end No. 1 Jelena Jankovic in Saturday's semifinals.

Serena, who completed Maroon Group play with a 3-0 record, including fighting off a match point to subdue sister Venus, will face Wozniacki in one semifinal. The 11-time Grand Slam champion held off Wozniacki, 6-7(5), 6-3, 7-6(3), in their lone career clash in Sydney in January.

Venus Williams takes on Jankovic in a rematch of the 2008 semifinals, which Venus won 6-2, 2-6, 6-3. Jankovic has won five of nine meetings with Williams.

The sixth-ranked Azarenka showed no signs of injury in the second set, but by the opening game of the third set both women were exchanging drop shots with regularity. Radwanska capped a running retrieval by flicking an exquisite backhand drop shot winner and held at love to open the third set. It was the sixth straight game she won.

At that point, Azarenka took an injury timeout for treatment on her left leg and lower back.

When play resumed, Azarenka held serve to stop the slide, but her movement was clearly compromised. Radwanska held at 30 for 2-1. On the ensuing changeover, Azarenka again took treatment from the trainer then, dropped serve to fall behind 1-3. Another backhand drop shot winner from Radwanska left Azarenka hobbling and pinching back tears. She limped to the net to shake hands, the latest casualty in a line of walking wounded to depart Doha.

The WTA Tour bills its season-ending Championships as the jewel of the calendar, but injuries and the oppressive heat combined to create a chaotic mess of a finale as three players — World No. 1 Dinara Safina, first alternate Vera Zvonareva and Azarenka — either withdrew or retired from the tournament.

Substitute medical bags for racquet bags, replace grips with gauze, exchange baseball caps for bandages and supplement the tournament staff with medical staff and you still couldn't remedy the various aches, ailments and injuries plaguing several of the top 10 women in the world.

A depleted Wozniacki, who suffered cramps in both legs that caused her to collapse during her grueling two hour, 48-minute victory over Vera Zvonareva on Thursday, had little left in the tank against Jankovic today.

The eighth-ranked Serbian broke serve five times in securing her semifinal spot with a 6-2, 6-2 victory in 68 minutes.

"I didn't really know what to expect from her today," Jankovic said immediately after the match. "I saw what happened to her yesterday but I just concentrated on trying to win the match and get through to the semifinals."

Venus can thank Svetlana Kuznetsova for her place in the semifinals. The reigning Roland Garros champion cracked seven aces and did not face a break point in rolling over fellow Russian Elena Dementieva, 6-3, 6-2, to eliminate Dementieva and send Venus into the final four.

"There was still a lot to lose today," Kuznetsova said. "Doesn't matter if I don't qualify. There are ranking points, prize money, everything... it's still a lot. When I go on court, I bring my name. I definitely don't want to lose, no matter what. I asked Venus before the match, 'Would you give me something if I win?' She was like, 'If you win, it's good for yourself!' "

Defending champion Venus has gone the three-set distance in all three of her matches. Venus, who suffered a three-set loss to Dementieva in her first match, held a match point against Serena before bowing 7-5, 4-6, 6-7(4). She bounced back to beat Kuznetsova, 6-2, 6-7(3), 6-4, yesterday.

Wozniacki admitted afterward she was emotionally deflated and physically fatigued after back-to-back punishing encounters that beat up on her body.

"After yesterday's match I was just totally dead. I got a little strain in my abdominal muscle and that didn't help much," Wozniacki said. "It was just tough luck. I got a lot of ice massage, drinking and eating and had so many bottles of fluid (after Thursday's match). I did everything I could... Definitely, I think I deserve to be in the semifinals after two great matches and such a big fight."

The inequity of the round-robin format was clear in that Jankovic, who looked listless in her 6-2, 6-3 opening-match loss to Azarenka on Tuesday, caught a break when current World No. 1 Safina retired from their White Group meeting on Wednesday after only two games. Meanwhile, Wozniacki was stretched to two hours, 58 minutes in rallying for a 1-6, 6-4, 7-5 win over Azarenka.

The physical toll of playing more than five hours in her two matches left Wozniacki looking a little spent today and Jankovic, playing crisp crosscourt combinations she sometimes disrupted with her favored two-handed backhand down the line, took full advantage.

Azarenka had punished Jankovic's serve on the opening day of round-robin play, but Jankovic won 26 of 31 first-serve points today and saved three of four break points.

"I tried to dictate the points and attack because I know how good she is in the long rallies," said Jankovic, who raised her record to 3-0 over Wozniacki in posting her first straight-sets win over the 19-year-old Dane.

Jankovic broke serve in the fifth and seventh games before serving out the set in 31 minutes.  Racing out to a 4-0 second-set lead, she never let Wozniacki have much say in the second-set rallies.