Swiss star Martina Hingis has brought an early end to her season after failing to recover from a hip injury.
Hingis has spent 209 weeks as world number one during her career
The 27-year-old former world number one was hoping to play in the Zurich Open in her home city of Zurich next week but has not been able to practise.
Her withdrawal has led to speculation in the Swiss media that she may be about to retire for a second time.
Hingis initially retired in 2003 because of persistent ankle problems before competing again two years later.
She said: "I'm very disappointed that I have to withdraw from the Zurich Open.
"I've done everything I can to be able to compete, but since Key Biscane I've had problems with my hip which are stopping me from playing any more tournaments this year and even from practising properly."
Hingis has won three title since her comeback, including a victory in Tokyo this year before the onset of her latest injury troubles.