Doha, Qatar -- (Sports Network) - Year-end No. 1 Serena Williams improved to a perfect 3-0 this week and secured a berth in Saturday's semifinals at the $4.55 million Sony Ericsson Championships.

The reigning Wimbledon and Australian Open champion Serena, seeded second here, handled fifth-seeded Olympic gold medalist Elena Dementieva 6-2, 6-4 at Khalifa Tennis Complex, as Dementieva dropped to 1-1 in the round-robin portion of this event.

Serena captured this prestigious tournament eight years ago.

Seventh-seeded and defending champion Venus Williams wound up at 1-2 in round- robin play, as she bested third-seeded French Open champion Svetlana Kuznetsova 6-2, 6-7 (3-7), 6-4 in just under two-and-a-half hours in Thursday's nightcap. Kuznetsova fell to 0-2 this week.

The Wimbledon runner-up Venus topped Russian Vera Zvonareva in last year's finale here in Doha.

Meanwhile, fourth-seeded U.S. Open runner-up Caroline Wozniacki overcame some leg cramps and moved to 2-0 for the week with a 6-0, 6-7 (3-7), 6-4 victory over the alternate Zvonareva (0-1).

Wozniacki needed a pair of medical timeouts to treat her cramping left thigh in the third set and collapsed to the court crying and grabbing both legs after netting a forehand during the final game. She would get back up to clinch the win when Zvonareva netted a forehand to end the bout.

Zvonareva, who saved two match points before ultimately succumbing on Day 3, required some medical attention of her own, for a bloody nose in the second set.

The 2008 Championships runner-up Zvonareva joined this year's exclusive eight- player field when top-seeded fellow Russian Dinara Safina pulled out of it on Wednesday because of a back injury. The French and Aussie Open runner- up Safina's exit guaranteed that Serena would close out 2009 as the top-ranked player in the world.

Serena had just lost the No. 1 ranking to Safina on Monday.

Round-robin play will wrap up with three more matches on Friday, as Kuznetsova will meet Dementieva, Wozniacki will battle eighth-seeded former world No. 1 Jelena Jankovic (1-1) and sixth-seeded Victoria Azarenka (1-1) will encounter Zvonareva.

This lucrative tournament caps the WTA season. A four-player Maroon Group boasts Serena, Kuznetsova, Dementieva and the former top-ranked Venus, while a four-woman White Group features Wozniacki, Azarenka, Jankovic and Zvonareva, playing in place of Safina.

An undefeated champ this week would collect $1.55 million.