Rafael Nadal of Spain returns the ball to Feliciano Lopez of Spain during their Italian Open tennis tournament match, in Rome (Photo: Reuters)
The start of the match was delayed by about 20 minutes as Nadal considered withdrawing.
"I had a fever last night and this morning I didn't feel better," he said. "I didn't know if I was going to go on the court for the match. I decided at the last moment." To keep his No. 1 ranking for at least one more week, Nadal will need to beat American Mardy Fish or Marin Cilic of Croatia in the quarterfinals. Otherwise, Novak Djokovic could claim the top spot by winning the tournament.
In his opening match Wednesday, Nadal struggled past 148th-ranked qualifier Paolo Lorenzi 6-7 (5), 6-4, 6-0.
That marked the third consecutive match in which Nadal dropped the first set, having also rallied to beat Roger Federer in the Madrid Open semifinals and then lost in straight sets to Djokovic in Sunday's final.
"I didn't understand why I felt so slow yesterday, why I had no energy. I understand today," Nadal said.
This is Nadal's fourth tournament of the clay season, having won titles in Monte Carlo and Barcelona before falling to Djokovic in Madrid. He was asked if his grueling schedule is wearing him down.
"I don't know," Nadal said. "That's something that's impossible to control. Usually I don't have this type of virus or fever and this is the second time this year." Nadal said his health could also be affected by his constant travel from one tournament to the next.
"That's not positive for the body. That's probably what's happening," he said.
This tournament is an important warmup for the French Open, which starts in 10 days. Nadal believes he'll be better by the time he gets back to Paris.
"I don't see why I won't be in perfect condition in one week and a half," he said.
Against Lopez, Nadal got out to an early lead in the first set, but then handed Lopez back a break to make it 4-3 by missing a routine forehand into the net. Lopez then held at love as Nadal didn't even attempt to run down one shot that appeared within reach.
Fortunately for Nadal, the 40th-ranked Lopez was not particularly sharp either, and made three consecutive errors before Nadal landed a backhand cross-court passing shot to break again and win the first set.
Nadal broke midway through the second set to take control for good.
In women's action on a cloudless day at the Foro Italico, Australian Open runner-up Li Na of China rolled past Jarmila Gajdosova of Australia 6-2, 6-1; two-time champion Jelena Jankovic edged Spanish qualifier Anabel Medina Garrigues 6-4, 2-6, 6-2; and last year's French Open finalist Sam Stosur beat Slovenian qualifier Polona Hercog 6-3, 6-4.
Federer, who has never won this title, meets Richard Gasquet of France later, and Djokovic attempts to extend his 35-match winning streak against 14th-seeded Stanislas Wawrinka in a night match.
Also later, top-ranked Caroline Wozniacki faces Yanina Wickmayer.