Roger Federer withdrew from the Madrid Masters on Monday because of a back injury.
The third-ranked Federer said that he was hurt in practice during the weekend and didn't want to take any chances aggravating the injury.
Federer was coming of a knee injury after the Australian Open which sidelined the Swiss player for about two months.
The 35-year-old Federer, a three-time winner in Madrid, had been a late addition to the field this week. The tournament was not originally on his schedule, but the former No. 1 decided to include it as part of his preparations for the French Open in three weeks.
Federer appeared fine in practice on Saturday, but cancelled his media appearance on Sunday and had not been back to the courts in the last couple of days, raising doubts about his participation.
He said he felt some pain while practicing on Saturday and it hadn't improved significantly since then, so he would not be fit to debut on Wednesday as scheduled.
Federer downplayed the seriousness of the injury, saying that it has happened to him before and he knows how to treat it.
He is still expected to play at the Rome Masters before heading to France for Roland Garros.
The Madrid Masters would have been Federer's second clay-court tournament of the season, and the first since he lost to Gael Monfils in the quarterfinals of the Monte Carlo Masters last month.
He lost to Nick Kyrgios in three sets in last year's second round in Madrid, a tournament he won in 2006, 2009 and 2012.
All other top-10 ranked players are playing in the men's tournament in Madrid this week.
Last week, Serena Williams, who was set to be the No. 1 seed in the women's draw, withdrew because of an illness. The tournament said that she had a fever and was feeling "less than 100 percent."
Williams won the Madrid tournament in 2012 and 2013.
Updated Date: May 02, 2016 20:19 PM