Doha, 30 October 2010 | AAP

Samantha Stosur faces a mighty challenge keeping her title hopes alive at the WTA Championships in Doha after being thrust into a semifinal against US Open champion Kim Clijsters.

Clijsters finished second in her round-robin group after losing her final pool match 6-4 7-5 on Friday to undefeated Russian Vera Zvonareva.

Stosur topped her group, meaning she will play Clijsters in the first semifinal on Saturday (1:30 am Sunday AEDT) at the Khalifa Tennis Complex.

Zvonareva will take on Dane Caroline Wozniacki in the second semifinal featuring the world’s top two players.

Stosur has lost all four previous encounters with Clijsters, most recently a three-set quarterfinal at last month’s US Open in New York.

That defeat was one of Stosur’s toughest of the season, the Australian squandering three services breaks in the deciding set and then having to watch on as the Belgian went on to land her third title at Flushing Meadows.

After winning their first three clashes in straight sets, Clijsters said Stosur’s progression this year made her a far more formidable opponent than previous seasons.

“Sam has always been a girl who has had a different type of play,” Clijsters said.

“I think she’s improved her strengths, which are her serve and forehand a lot. I think her footwork and speed has improved a lot. That was definitely one of her weaknesses in the past when I played her before.

“Now she’s definitely found a game that I think really, really suits her. She feels a lot more comfortable being out there. I think that has shown in her results as well.”

Clijsters said she felt flat against Zvonareva following a draining three-set win on Thursday over Victoria Azarenka.

“Obviously last night’s match was pretty tough and I kind of just felt it today,” the three-time grand slam champion said.

“So obviously I’m going to have to feel much better and have to get my energy back. I kind of just felt flat out there today.

“I have the morning off, kind of, and just around four o’clock I’ll practice. I’m going to really use that time to recover and to be as fresh as possible.”

Meanwhile, Russian star Elena Dementieva earlier on Friday shocked the tennis world by announcing her retirement after bowing out of the tournament.

The 29-year-old world No.9’s final match ended in a 6-4 6-2 loss to French Open champion Francesca Schiavone.

The Beijing Olympic gold medallist and dual grand slam runner-up had scored her 576th and last tour win against Stosur on Thursday.

Dementieva reached a career-high No.3 in the world only last year and retires as the best player of her generation never to win a major.