Doha, Qatar -- (Sports Network) - Russian Vera Zvonareva upset world No. 1 Jelena Jankovic to win her round-robin group, while Serena Williams pulled out here on Friday, giving two-time major runner-up Elena Dementieva a walkover into Saturday's semifinals at the season-ending $4.55 million Sony Ericsson Championships.

The eighth-seeded Olympic bronze medalist Zvonareva went a perfect 3-0 in this week's four-woman White Group, including a big 2-6, 6-3, 6-4 come-from-behind victory over the U.S. Open runner-up Jankovic on Day 4 at Khalifa Tennis Complex. Jankovic, who went 2-1 in round-robin play and will finish this year as the No. 1 player in the world, will also compete in Saturday's semis.

Zvonareva's final four opponent will be fifth-seeded fellow Russian Elena Dementieva, while Jankovic will take on seventh-seeded American Venus Williams, who was a flawless 3-0 in capturing this week's four-player Maroon Group.

The Olympic gold medalist Dementieva was scheduled to meet a third-seeded Serena here on Friday with a semifinal berth on the line, but the U.S. Open champion from the United States pulled out, citing a stomach muscle injury. The nine-time major champion Serena lost to her big sister and fellow former world No. 1 Venus, 7-5, 1-6, 0-6, here on Thursday.

The 27-year-old Serena, who was 1-1 in the Maroon Group this week before pulling out against Dementieva, was the only former Championships titlist (2001) in this year's field and was the runner-up in 2002 and 2004.

Dementieva wound up going 2-1 in the round-robin portion, as she got past fellow Russian Nadia Petrova 6-4, 4-6, 6-4 in a meaningless affair on Friday. Petrova played in place of the ailing Serena as a second alternate.

In the final round-robin match of the week, first alternate Agnieszka Radwanska of Poland handled sixth-seeded Russian Svetlana Kuznetsova 6-2, 7-5.

Radwanska replaced fourth-seeded French Open champion and Aussie Open runner- up Ana Ivanovic, who went 0-2 this week while suffering with a viral illness. Ivanovic pulled out on Friday instead of facing Kuznetsova, who was a dismal 0-3 for the week.

The Maroon Group was rounded out this week by 0-3 Dinara Safina, the Russian third seed that had runner-up finishes at the French Open and Olympic Games this year.

On Saturday, Jankovic and the reigning two-time Wimbledon titlist and seven- time major champion Venus will meet for a ninth time, with the Serb holding a 5-3 lifetime advantage, including a 2-0 record this year. The two stars have never met at the season-ending Championships.

Meanwhile, Dementieva and Zvonareva will do battle for a sixth time, with Dementieva leading their all-time series, 4-1. Dementieva is 2-1 this year, with the wins coming at the French Open and the semifinals at the Summer Games.

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