Russia's Dinara Safina hobbled out of the WTA Championships last night, surrendering the year-end World No 1 ranking to Serena Williams.

Williams then celebrated the landmark by beating sister Venus 5-7, 6-4, 7-6 (7-4), sending the defending champion to a second defeat in two days which leaves her on the brink of exiting the 4.55 million dollar tournament.

The 23-year-old Safina quit her opening match against Jelena Jankovic with a lower back injury with the score at 1-1 in the first set .

"My body just gave up. I couldn't handle the pain anymore," said Safina.

"I was advised to take a break after the US Open. But I was chasing this No 1 place and fighting with my body, added Safina, whose earlier 25-week reign as No 1 had come under fire due to the absence of a major title on her CV.

Meanwhile, Serena was delighted to be No 1. "It's a great feeling but I feel really sorry for Dinara. She's a great player," said Serena.