Doha, Qatar -- (Sports Network) - Serena Williams opened with a win, while her big sister Venus came up a round-robin loser Tuesday at the season-ending $4.55 million Sony Ericsson Championships.

A second-seeded Serena edged out third-seeded French Open champion Svetlana Kuznetsova 7-6 (8-6), 7-5, while a seventh-seeded Venus lost to fifth-seeded Olympic gold medalist Elena Dementieva 3-6, 7-6 (8-6), 6-2 on opening day at this prestigious eight-player event at Khalifa Tennis Complex. Venus had won six straight matches against the two-time major runner-up Dementieva.

Venus is the reigning Championships titlist, having beaten Russian Vera Zvonareva in last year's title match here.

In one other round-robin affair on Day 1, sixth-seeded Victoria Azarenka whipped eighth-seeded former world No. 1 Jelena Jankovic 6-2, 6-3.

The Wimbledon and Australian Open champion Serena and Aussie and French Open runner-up Dinara Safina will decide who finishes the as the year-end No. 1 on the WTA Tour this week. Safina just regained the No. 1 ranking from Serena on Monday.

If Serena can win one more match than Safina this week, the American will finish the year No. 1 for the second time in her career. If the two women win the same amount of matches, or Safina wins more, the Russian will finish as the year-end No. 1 for the first time in her career.

This exclusive tournament caps the WTA season. A four-player White Group features a top-seeded Safina (0-0), fourth-seeded U.S. Open runner-up Caroline Wozniacki (0-0), Azarenka (1-0) and the former No. 1 Jankovic (0-1). The four-woman Maroon Group boasts Serena (1-0), Kuznetsova (0-1), Dementieva (1-0) and the former top-ranked Venus (0-1). Serena captured this prestigious event eight years ago.

Three more round-robin matches will be staged here on Wednesday, as Safina will meet Jankovic, Serena will battle Venus and Wozniacki will take on Azarenka. Wozniacki and Azarenka are making their Championships debuts this week.

Round-robin play will determine Saturday's semifinalists, and the champion will be decided in the lucrative final here on Sunday.

An undefeated champ this week would collect $1.55 million.