Russia's Maria Sharapova reacts during the first round match against Polands' Agnieszka Radwanska at the WTA Championships in Istanbul of Turkey October 24, 2012. [Photo/Agencies]

ISTANBUL?- Victoria Azarenka moved a step closer to securing the year-end number one ranking with a 6-7 7-6 6-4 win over Germany's Angelique Kerber at the WTA Championships on Wednesday.

Azarenka needed three hours and six minutes to repel Kerber but Maria Sharapova's 5-7 7-5 7-5 win over Agnieszka Radwanska took even longer to complete, the three hours and 12 minutes contest ending at 02:12 on Thursday morning.

In another tight contest that opened the evening session, number three Serena Williams took down China's Li Na 7-6 6-3 in one hour and 50 minutes.

Before play began, title holder Petra Kvitova of the Czech Republic pulled out of the tournament due to a viral illness and was replaced by Australian Samantha Stosur.

With her round robin victory in the Red Group, Azarenka only needs to win one more match to secure the year- end top spot.

With their second wins of the Championships, Sharapova and Williams both guaranteed themselves spots in the semi-finals.

Belarusian Azarenka fought off two match points at 6-5 down in the second set in brilliant fashion, ripping an inside-out forehand and backhand down the line winners.

She won the tiebreaker 7-2 with another forehand winner and managed to stop a Kerber charge in the third set.

The hard hitting Azarenka failed to serve out the match at 5-3, but broke the German to win their encounter with a forehand down the line.

"It was definitely a great fight," Azarenka said. "I'm more proud of the way I handled and stayed in that match."

Sharapova was a set and 2-4 down in the second before storming back to best the Pole, despite having her serve broken five times in the match.

"Today was an example of not playing my best tennis at all, fighting through it, hanging in there, and getting it done," Sharapova said.

Li and Williams both struggled with their serve, and the American became so frustrated after double faulting twice in the fourth game of the first set that she smashed a racket and received a warning.

"I guess I got angry and I wasn't able to control myself," she said. "But sometimes I play better after I get really angry."

After winning the first set tiebreaker 7-2 on a forehand return of serve, Williams broke the former French Open champion to 3-1 in the second set.

Li managed to fight off two match points to hold at 3-5, but Williams won the contest on her third match point when Li erred on a forehand.

"I think she has a good return, but I don't know how she can return a ball that's in the net or out," said Williams, who was broke five times in the match.

"I think most of my serves were either in the net or out. I lost serve today more than in all of Wimbledon."

Williams will meet Azarenka later on Thursday in a repeat of this year's US Open final, which Williams won.