(From left to right) China's Li Na, German Angelique Kerber, Petra Kvitova from the Czekh, Victoria Azarenka from Belarus, Russia's Maria Sharapva, Serena Williams of the US, Poland's Agnieszka Radwanska and Sara Errani of Italy pose for a group photo on the eve of the season-ending WTA Championships in Istanbul of Turkey October 21, 2012. [Photo/CFP]

ISTANBUL - Victoria Azarenka looks favourite to end the year as world number one but few of the eight women who qualified for this week's season-ending WTA Championships would argue that Serena Williams still rules the roost.

Williams, the 30-year-old American ranked third in the world, won the last two grand slams of the season, either side of claiming singles and doubles gold at the Olympics.

With her injury problems and health issues behind her, she has appeared all but unbeatable in the second half of the season.

She has not played a match since beating Azarenka to win her 15th grand slam title at Flushing Meadows but will be re-charged for the start of the Championships in Istanbul on Tuesday, where Williams faces Azarenka, China's Li Na and German Angelique Kerber in round-robin action.

The other group features last year's season-ending champion Petra Kvitova, Maria Sharapova, Sara Errani and Agnieszka Radwanska.

The sensible money will be on Belarussian Azarenka and Williams taking the top two spots in their group and going on to meet in next Sunday's final.

"I think Serena is for sure this summer the best player in  the world, winning two grand slams and the Olympics in between," fourth-ranked Radwanska of Poland, runner-up to Williams at Wimbledon, told reporters on the eve of the tournament.

"For sure she's on fire."

Williams, who has not played at the season's finale since losing in the final to sister Venus in Doha in 2009, will begin against tournament debutante Kerber.

"I don't really care who I play or when I play, as long as I can play," said Williams, who completed the so-called golden slam by winning the Olympic singles title in London.

"I'm really happy to be here. I'm on the red side with Victoria, so it'll be good."

Li will also find herself in Williams's sights, and she expects the American to be tough to beat if she is at the top of her game. "I always say, if she is 100 percent on court, nobody can beat her," seventh-ranked Li said.

Williams and Azarenka lead the tour with six titles apiece this year but Williams has lost only four matches in 2012 and only one since the French Open.

French Open champion Sharapova, thrashed by Williams in the Olympic final, also pointed to the American as the year's outstanding player, whatever happens this week.

"Definitely winning two majors and the Olympics, for sure," said the Russian, who could snatch the number one ranking if she wins the tournament and Azarenka fails to win two matches.

Kvitova, who has struggled to scale the heights she managed last year when she won Wimbledon, believes the title of the year's best player will boil down to this week.

"Vika was unstoppable in the beginning of the season," Kvitova said. "Then Maria became number one, she won the French Open. Then Serena had amazing summer."

(From left to right) Belarus' Victoria Azarenka, Russia's Maria Sharapva and Serena Williams from the US pose for a photo call on the eve of the season-ending WTA Championships in Istanbul of Turkey October 21, 2012. [Photo/CFP]