DOHA: Serbia's Jelena Jankovic reached the semi-finals of the 4.55 million dollar WTA Championships on Friday beating Caroline Wozniacki whose injury-hit campaign returned to haunt her.

Jankovic, who made the semi-finals in 2008 just before her elevation to the world number one spot, comfortably beat the US Open runner-up and world number four 6-2, 6-2 in just 68 minutes.

She came into the match far fresher than her teenage opponent.

The 24-year-old Serbian, defeated in her opener by Victoria Azarenka, had benefitted from spending just 95 minutes in total on court in two matches after Dinara Safina withdrew through injury from her second tie after only 12 minutes.

In stark contrast, Wozniacki had spent a total of 5hr 48min seeing off Azarenka and then Vera Zvonareva, only surviving the clash with Russian after overcoming a crippling attack of cramps.

"I saw Caroline's match on Thursday. I wanted to reach the semi-finals and I didn't want to get into long rallies with her," said Jankovic after her third win in three matches against the 19-year-old Dane.

Wozniacki can still make the semi-finals but she needs Poland's Agnieszka Radwanska, who has replaced the injured Zvonareva, to beat her best friend Azarenka later Friday.

In the day's other match, Elena Dementieva needs to beat Russian compatriot Svetlana Kuznetsova to make the last four. A defeat would allow Venus Williams to sneak through with sister Serena.