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.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.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.