Doha, Qatar -- (Sports Network) - Reigning No. 1 Jelena Jankovic of Serbia and former top-ranked star Venus Williams of the United States were a pair of round-robin winners Thursday at the season-ending $4.55 million Sony Ericsson Championships.

The U.S. Open runner-up Jankovic (2-0) secured a berth in Saturday's semifinals with a 7-6 (8-6), 6-4 victory over sixth-seeded Russian Svetlana Kuznetsova, while a seventh-seeded Williams (3-0) also landed in the final four here by coming back to paste her younger sister Serena 5-7, 6-1, 6-0 at Khalifa Tennis Complex. The 0-2 Kuznetsova has been eliminated from semifinal contention.

Jankovic, who heads up this week's four-player White Group, has already clinched the year-end No. 1 ranking for the first time in her career.

The nine-time major champion and former No. 1 Serena, seeded third this week, is now 1-1 in the Maroon Group and still in contention for a spot in the semis. The seven-time major titlist Venus has won the group by virtue of her perfect record.

Venus defeated the U.S. Open champion Serena in July's Wimbledon final and the sisters have now split 18 career matchups on the WTA Tour, including a 2-2 mark this year. Oddly enough, this marked their first-ever meeting at the season-ending Championships.

The 27-year-old Serena is the only former Championships titlist (2001) in this year's field. She was also the runner-up in 2002 and 2004.

Round-robin tennis is the order of the day here through Friday, with the top-two players from each group advancing to Saturday's semifinals and the semifinal victors moving into Sunday's lucrative finale.

The winner of this week's event will collect at least $1.34 million.