Dinara Safina pulled out of the Sony Ericsson Championships after just two games with a serious back injury, handing the year-end No. 1 ranking to Serena Williams.

Safina said a disc in her lower back was “starting to fracture” and the injury has been bothering her for three months. She said doctors told her she will be sidelined for at least six weeks, and that she may not be fit in time for next year’s Australian Open.

The Russian said she took anti-inflammatory injections to try to defend her No. 1 ranking in Doha.

“I went yesterday to have injections, cortisone,” she said. “But I just couldn’t handle this pain.”

Safina, the main No. 1 since April, regained the top ranking this week, but her margin over Williams was so slim that the player who performed best in Doha was guaranteed to end the year as No. 1.

Williams won her first match on Tuesday and beat sister Venus 5-7, 6-4, 7-6 (4) on Wednesday.

“It’s such a great feeling,” Williams said about being back at No. 1. “Dinara is such a great player. She must be really hurt, because she never gives up.”

Williams, the Australian Open and Wimbledon champ, will end a year as No. 1 for the first time since 2002.

Safina was serving at 1-1 against Jelena Jankovic when she stopped play, walked over to her chair and covered her face with a towel. She then told the chair umpire she could not continue.

She was replaced in the White Group by compatriot and first alternate Vera Zvonareva, last year’s runner-up to Venus Williams.

“When I speak with the doctors, it doesn’t sound so good,” Safina said. “It is possible I may have to skip it.”

Serena Williams led Venus by 4-1 and two breaks but let her come back and had to save a match point when serving at 6-5. Serena then took a 3-0 lead in the tiebreaker and converted her first match point with an ace, 12 minutes past midnight local time in front of a sparse crowd at the Khalifa Tennis Complex.

The tiebreaker was interrupted briefly at 5-3 when a cat ran across the court and had to be chased toward one of the exits.

It was defending champ Venus’ second straight loss, but she still has a chance to reach the semifinals if she beats Svetlana Kuznetsova on Thursday and Serena defeats Elena Dementieva in the Maroon Group.

Earlier, Caroline Wozniacki saved a match point in the final set and beat Victoria Azarenka 1-6, 6-4, 7-5.

Wozniacki, who had just two winners in the first set, saved a match point with Azarenka leading 5-4 in the third before finally holding serve for the first time in the decider.

“I just felt like I hadn’t lost the match yet,” Wozniacki said. “She still had to win one more point.”

Wozniacki broke again for 6-5 after the Belarusian was docked a point for smashing her racket. Azarenka had already been warned in that game after hitting the ball out of the court following another unforced error.

It was her first match in the tournament, while Azarenka defeated Jankovic in her opener and remained in contention for the semifinals.

Each group stage win is worth $100,000, with the eventual winner taking home up to $1.55 million.

Wozniacki seemed bothered by the sore hamstring that forced her to retire from her first-round match at the Luxembourg Open last week. She wore strapping around her left thigh, icing it during changeovers and stretching repeatedly between points.

“The strapping definitely helped,” she said. “You try not to think about it too much, especially in the important moments. ... It’s not great for the leg, running this much.”

More In: Tennis | Sport