Serena Williams continued to show why she will end the year as number one as she beat Elena Dementieva in Doha.

The win sealed her palce in the semi-finals of the Sony Ericsson Championships.

Williams needed to show resilience after falling a break down in the first set but then won seven games on the spin and never looked back from there - although she needed three match-points to finish it off.

"It was definitely a match I had to be really focused for, because she's a really good player," Williams told www.wtatour.com.

"She's one of those late bloomers who tends to get better and better as every year goes on in her career. I was probably playing the fittest girl on tour."

As well as booking a place in the last four for Williams, who will finish the year as number one after current leader Dinara Safina was forced to withdraw through injury yesterday, the result also gave her sister Venus a chance of progressing.

Venus made the most of that opportunity by beating Svetlana Kuznetsova 6-2 6-7 (3/7) 6-4 in this evening's late match.

The second qualifying place from the Maroon Group will now be decided by tomorrow's match between Dementieva and Kuznetsova.

In the White Group, Caroline Wozniacki came through another three-set marathon to beat Vera Zvonareva 6-0 6-7 (3/7) 6-4.

Wozniacki was 5-2 up in the second set and seemingly on the way to a comfortable victory against the player who took Safina's place in the draw.

Zvonareva battled back, however, taking the second set - and all the momentum - as the players went off for a break because of the hot-weather rule.

When Wozniacki started to cramp upon resumption, it seemed all over for the fourth seed - but she somehow battled through and broke down in tears after claiming the winning point.

"I feel great that I won under these conditions," she said.

"I have absolutely no idea how I pulled it through. But I'm very happy about it."