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Serena back on top

Serena Williams has become the oldest women's world number one after her three-set win over Petra Kvitova in the Qatar Open quarter-finals.

Victory for the 31-year-old American ensured she will eclipse the previous oldest mark set by Chris Evert, who was 30 years and 11 months when she was number one in November 1985.

It marks Williams' return to the top spot after a gap of almost two-and-a-half years, having been usurped by Caroline Wozniacki in October 2010, and her fourth reign at the top in total.

The feat also caps a remarkable comeback from Williams, who missed almost an entire year on tour due to injury and illness in 2011, and briefly slipped as low as 175th in the world.

Williams scotched retirement rumours by storming back in 2012 to win Wimbledon, US Open and Olympic titles.

Today she fought back from going a set behind and triumphed 3-6 6-3 7-5 in just over two hours, despite also having twice fallen a break behind in the final set.

She will next face Maria Sharapova in the semi-finals, after the Russian third seed beat Australia's Samantha Stosur 6-2 6-4 in an hour and 22 minutes.

Top seed and now-deposed number one Victoria Azarenka moved into the semi-finals with a 6-2 6-2 win over Italy's Sara Errani.

Williams said of her spirited victory: "I don't know how I did it - I really don't know. I just hung in there and she was playing so well. Every time I looked around she was hitting a winner. I just tried to stay in there."

The journey back to the top of the rankings has been particularly difficult for Williams.

She survived emergency surgery in Los Angeles in March 2011 - a haematoma and a pulmonary embolism having put her life in the balance - as she admitted at Wimbledon last year.

In tears today, she said: "I never thought I would be here again. I've just been through so much and never thought I'd be here again."

After Williams' win, the final match of the day in Doha saw Polish fourth seed Agnieszka Radwanska beat Danish 10th seed Wozniacki 6-2 7-5 to set up a last-four tussle against Azarenka.