Doha, Qatar -- (Sports Network) - Serena Williams outlasted her big sister Venus on Wednesday and will now end the year at No. 1, while currently top- ranked Dinara Safina had to retire from her opening round-robin match and was forced to withdraw from the season-ending $4.55 million Sony Ericsson Championships.

Serena's two victories already this week, combined with Safina's withdrawal, assures that the American star will finish as a year-end No. 1 for only the second time in her illustrious career. The 28-year-old Serena also turned the trick back in 2002.

The reigning Wimbledon and Australian Open champion Serena just lost the No. 1 ranking to Safina on Monday.

"2009 has been a truly memorable season for me during which I enjoyed some of the biggest wins of my career," Serena said. "Capturing the year-end world No. 1 ranking is a huge accomplishment for me and I am thrilled that all the hard work has paid off."

The second-seeded Serena (2-0) edged out the seventh-seeded Venus (0-2) in a tough three-setter, 5-7, 6-4, 7-6 (7-4), on Day 2 here. Venus is the defending champion here, having beaten Russian Vera Zvonareva in last year's title tilt.

The 2001 Championships titlist Serena needed 2 hours, 41 minutes to prevail and swatted 11 aces en route to victory. Venus helped her little sister's cause by piling up 10 double faults in a sloppy match that featured 14 service breaks, seven for each star.

Meanwhile, a top-seeded Safina (0-1) was tied with eighth-seeded former world No. 1 Jelena Jankovic (1-1) when the Russian quit with a back injury at 1-1 in the first set at Khalifa Tennis Complex.

The French and Aussie Open runner-up Safina was serving when she stopped playing, walked over to her chair and covered her face with a towel. She informed the chair umpire that she was unable to continue.

Safina said the bone structure in her lower back is "starting to fracture" and she's been battling the injury for three months. She said doctors told her she will be out of action for at least six weeks, and she may not be ready for January's Australian Open.

"I went yesterday to have injections, cortisone," the Russian said. "But I just couldn't handle this pain."

One other match on Day 2 here saw fourth-seeded U.S. Open runner-up Caroline Wozniacki (1-0) stave off a match point in order to come from behind and beat sixth-seeded Victoria Azarenka 1-6, 6-4, 7-5. Azarenka is 1-1 so far at this prestigious eight-player event.

Wozniacki and Azarenka are making their Championships debuts this week.

This exclusive tournament caps the WTA season. A four-player White Group featured Safina, Wozniacki, Azarenka and Jankovic. The four-woman Maroon Group boasts Serena, French Open champion Svetlana Kuznetsova (0-1), Olympic gold medalist Elena Dementieva (1-0) and the former top-ranked Venus. Safina has been replaced in the field by the first alternate, her fellow Russian Zvonareva.

On Thursday, Serena is slated to face Dementieva, Kuznetsova will lock horns with Venus and Wozniacki will be opposed by Zvonareva, who failed to qualify for one of the top-eight spots this week, but has now filled Safina's vacated spot.

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.