Roger Federer has pulled out of this week's Madrid Open with a back injury as his troubled season continues.
The 34-year-old told a press conference that he suffered the problem in practice on Saturday and described his withdrawal as a precaution.
Federer, who had a first-round bye in Madrid, is still hoping to play in next week's Masters series event in Rome, which is the final major tournament in the build-up to the French Open starting on May 22.
One of the hallmarks of the 17-time grand slam champion's longevity has been his ability to stay healthy but it has been a different story this year.
Federer needed surgery for the first time in his career in February after hurting his knee running a bath for his twin daughters.
That put him out of action for nearly two months, and his planned comeback at the Miami Open had to be postponed after he picked up a stomach bug.
Federer did return at the Monte-Carlo Masters last month, reaching the quarter-finals, where he lost to Jo-Wilfried Tsonga.
Back problems have been his one area of physical weakness but he described this latest recurrence as not as serious as the one that forced him to pull out of a scheduled final against Novak Djokovic at the ATP World Tour Finals in London in 2014.
Federer only decided to play in Madrid last week and the news is a blow to the tournament, which also lost Serena Williams to illness before the start.
Despite not playing, Federer may well return to world number two next week, with current incumbent Andy Murray defending the title he won with victory over Rafael Nadal 12 months ago.