Nadal was the defending champion at the clay-court event.
"My doctor said I need a few days off,'' Nadal said. "I will have a checkup and treatment and won't return to the court until I am 100 percent fit.''
The field, hit by several other late withdrawals, is now led by No. 11-ranked Nicolas Almagro of Spain and No. 15 Richard Gasquet of France.
Two Germans who live near the tennis courts advanced to the second round. Michael Berrer beat Eric Prodon of France 6-3, 6-4 and Andreas Beck beat training partner Daniel Brands 3-6, 6-3, 6-4.
Nadal dethroned Roger Federer as Wimbledon champion Sunday, needing 4 hours, 48 minutes to hold off the Swiss star's stirring comeback 6-4, 6-4, 6-7 (5), 6-7 (8), 9-7.
Also, Juan Martin Del Potro of Argentina beat Evgeny Korolev of Russia 6-3, 7-5 and Oscar Hernandez won a Spanish duel against Santiago Ventura 6-1, 6-4.
JIMENEZ WITHDRAWS FROM WALES OPEN
Federer, Nadal start slow, then roll at French
IMMELMAN WITHDRAWS DUE TO ILLNESS
