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.
Radwanska, Kerber lose
Top-seeded Agnieszka Radwanska and Australian Open champion Angelique Kerber were upset in the opening round of the Madrid Masters on Sunday.
Radwanska, who was elevated to the No 1 seeding when Serena Williams withdrew, was beaten 4-6, 7-6 (3), 3-6 by Dominika Cibulkova of Slovakia.
Radwanska hit five aces but also had six double-faults and was broken 11 times by Cibulkova.
Barbora Strycova of the Czech Republic beat second-seeded Kerber 6-4, 6-2.
Cibulkova was coming off a win at the Katowice Open last month and a narrow loss to Radwanska at Indian Wells, where she had a match point.
Cibulkova will next meet Caroline Garcia, who was leading 6-4, 2-1 when Johanna Konta retired from their match.
The 33rd-ranked Strycova faces Madison Keys, who rallied past fellow American Alison Riske 1-6, 6-3, 6-4.
Third-seeded Garbine Muguruza had little trouble advancing past Anna Karolina Schmiedlova 6-2, 7-5, breaking her serve six times.
Defending champion Petra Kvitova eased past lara Arruabarrena 6-3, 6-2, and fourth-seeded Victoria Azarenka defeated laura Robson 6-4, 6-2. Simona Halep downed Misaki Doi 6-0, 6-3, and Carla Suarez Navarro had a 3-6, 6-2, 6-3 win over Timea Babos.