Dinara Safina quit her WTA Championship opener against Jelena Jankovic in tears on Wednesday after suffering a back injury which could see her lose her world number one ranking.

The emotional pullout, at just 1-1 in the first set, could now allow number two Serena Williams to retake the year-end top spot if the Russian is forced to withdraw from the rest of the 4.55 million dollar tournament.

Safina went back to the top of the rankings on Monday, but only with a 155 point lead on the American.

Williams won her opener on Tuesday against Svetlana Kuznetsova, to earn 160 points, and was facing sister Venus later on Wednesday.

Should Safina be forced to withdraw from her remaining two group games against Victoria Azarenka and Caroline Wozniacki, then Williams will reclaim the year-end top spot.

- It's really unfortunate. Dinara said she had hurt her lower back - said Jankovic after the 13-minute match.

- I wished her well for her recovery. We all have problems with our bodies at this time of the year. -

Safina's retirement leaves the White Group evenly-poised with Jankovic, Wozniacki and Azarenka all with one win with the top two making the semi-finals.

The Maroon Group in the tournament, which features the world's top eight players, is made up of the Williams sisters, Kuznetsova and Elena Dementieva.