Afterwards, Safina said a disc in her lower back is starting to fracture.

“I've been in pain since Portoroz. I've been using anti-inflammatories to play with this pain for about three months,” she told the tournament’s official website.

A long lay-off for at least six weeks is needed for the 23-year-old. Serena Williams will now reclaim the World No. 1 ranking.

Williams sisters clash once more

Serena took on her elder sister and defending champion Venus Williams in the Maroon Group, and the Americans did not disappoint, producing some amazing tennis in front of the Doha crowd.

Venus took the opener 7-5, but Serena came back in the second 6-4, and made a strong start in the decider.

She won two-breaks and took a 4-1 lead, but Venus turned things upside down, winning five of the next six games to lead 6-5, and held match point.

Serena played that vital point aggressively and finished it off with a massive overhead. She than leveled the score at six-all, assuring the game will be decided on tie break, which she won.

The match ended 5-7, 6-4, 7-6 for Serena, while Venus, who has lost two in a row, may now forget about the playoffs.

“It really could have gone either way. At the end of the third set she just started playing great. I was just trying to do what I could. I kept saying: I want to end this year No.1, so I’d better lift up the level of my game,” Serena Williams commented.

In another Wednesday match, two debutantes of the season-ending tournament clashed in the White Group.

Caroline Wozniacki of Denmark needed nearly three hours to take the better of Belarusian Victoria Azarenka, 1-6, 6-4, 7-5.