DOHA (AFP) Caroline Wozniacki must wait at least one more day to become year-end world number one, after suffering a surprise defeat at the round robin stage of the WTA Championships here Wednesday. The in-form favourite, who had previously lost only two out of 33 matches and won five titles, was beaten 6-4, 6-3 by Samantha Stosur, the world number five from Australia. Wozniacki had a great chance to reach break point in the ninth game of the first set, but after missing a relatively easy kill started to play as though under pressure. Stosur by contrast hit freely, striking 26 winners, serving excellently, and opening up the court frequently with her forceful inside-out forehand. Wozniacki denied suggestions that she allowed the year-end number one spot to enter her mind. If she beats Francesca Schiavone, the French Open champion from Italy, in her next match it will happen and maybe even if she doesnt, but qualifies from the group with one win. Wozniackis difficulties began at four-all, 30-all, when she had the court at her mercy but from a position close to the net, drove the ball long. She hurled down her racket hard in annoyance. The following game saw Wozniacki serve a double fault to go set point down, and on her first big opportunity Stosur played a great point. She imposed a difficult angle upon the Wozniacki backhand, and strode forward to finish the rally with a rousing drive volley. Stosur broke serve again for 3-1 in the second set when Wozniackis straight backhand drive veered off the parallel into the tramlines. It was damage which the favourite never repaired. She did though get to break back point at 30-40, only for Stosur to deliver a fierce serve and follow up with a drive-kill, then closing out the match with the help of two more heavy first serves. Earlier Kim Clijsters made a flawed but satisfactory start to her bid to win back the WTA Championship title she last won seven years ago, scoring a straight sets win over Jelena Jankovic. Even though Clijsters had not competed since retaining her US Open title six weeks ago in New York, she moved well and competed confidently in a 6-2, 6-3 win over the former world number one from Serbia. There were few obvious signs of discomfort in the foot from which a mole was removed in an operation last month, though her tally of double faults did creep to a surprisingly high total of ten. But Clijsters hit the ball well from the forehand side and struck 25 winners altogether, and in the big points she played well. It was my first match back, and I was happy not with everything but Ill take that, she said. I need to be a little more consistent and deliver fewer double faults, but I will try to be better tomorrow (Thursday). Jankovic again looked below par with the sinus infection she sustained last week, though she played better than she had against Vera Zvonareva on Tuesday, after which, she claimed, she collapsed in the changing room. Earlier Zvonareva made an important step forward to reaching the final for the second time by winning a long and often superbly contested match with Victoria Azarenka 7-6 (7/4), 6-4. After that the size of Zvonarevas smile indicated how hard the contest had been. Two sets lasted fully two hours and 17 minutes.