Serena's hot streak continues in Istanbul

(CNN) -- She might be the hottest female player on the planet but Victoria Azarenka seemed powerless to arrest her miserable run of form against Serena Williams at the WTA Championships in Turkey.

The world No.1 from Belarus has won just one solitary match against the 15-time grand slam winner from America and went down to a 6-4 6-4 defeat in her second round robin outing.

For Williams, 31, it represented her third straight win in Istanbul, guaranteeing her a place in the final four of the season-ending $4.9 million tournament.

Azarenka, who saved two match points before beating Germany's Angelique Kerber in an epic clash on Wednesday night, must now beat China's Li Na on Friday to make the semifinals.

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And if the 23-year-old needed any extra motivation, victory should see Azarenka cement her world No. 1 status for the end of the 2012 season.

"I have been practicing hard and expected to play better," Azarenka said in a court side interview. "Hopefully as time goes on, I will get better. If not, I hope to play better next year.

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"When I did the right things I was in control much of the time, but I didn't have the consistency today. I had too many opportunities I didn't use. I will be a little bit sad, but I need to bounce back straight away.

"I will give everything to win (on Friday). I will give two hundred percent. I have another opportunity to stay in the tournament and to keep the dream alive."

Defeat snapped a 13-match winning streak for Azarenka after the U.S. Open but Williams is now unbeaten in 16 clashes against the world's top five players.

The American had already sealed her place in the last four with wins over Li Na and Germany's Angelique Kerber.

Li, the tournament's lowest ranked player, defeated Kerber, the No. 5 seed, 6-4 6-3 in Thursday's opening match to keep alive her hopes of making the semis.

Li responded to her defeat to Serena Williams by brushing aside Kerber, who was knocked out of the tournament after her epic encounter with Azarenka on Wednesday evening.

Kerber has now lost all three of her matches, winning just one set in the process, while Li can upset the odds and make the semis by beating Azarenka on Friday.

In the late match on Thursday, Italy's Sara Errani responded after her opening defeat to Agnieszka Radwanska by toppling Australian Sam Stosur 6-3 2-6 6-0.

Meanwhile, less than two weeks from the start of the men's season-ending ATP World Tour Finals in London, Rafael Nadal has announced his withdrawal with injury.

The 11-time grand slam winner from Spain hasn't featured on the men's Tour since his surprise defeat to Czech Lukas Rosol in the second round at Wimbledon back in late June.

Nadal, still holding on to his position as world No. 4, will also miss the Paris Masters event prior to London and is targeting a return at the Australian Open in early 2013.

He said: "It's disappointing for me to miss the last two tournaments of the season in Paris and London, but it doesn't come as a surprise.

"I will hopefully resume my tennis practice soon since I am making good progress with my recovery from injury. I am not ready to compete in time for these events so I will continue my recovery in Mallorca and work hard to be back as soon as possible."

Nadal's spot in London will be taken by Juan Martin Del Potro, who assured his place with a 7-5 6-1 defeat of American Brian Baker at the Swiss Indoors event in Basel.

And though the Argentinean was delighted to make the cut, he expressed his sympathy to Nadal on the ATP Tour's official website.

He said: "I'm sad for Rafa, he's really trying in his comeback. But it's not easy after such a long time away. I had the same feeling with my wrist.

"We miss Rafa a lot on the Tour. He is one of the best players in history, he will come back strong. I'm sure it will be very soon. He will be ready to win a big event again."