Nadal has been virtually unbeatable on clay
Rafael Nadal suffered only his second defeat in 105 matches on clay when he lost 7-5 6-1 to Juan Carlos Ferrero in the second round of the Rome Masters.
The world number two, who was seeking to win his fourth successive title in Rome, struggled with a blister on his foot and needed treatment on court.
The loss comes after Nadal complained recently of an overcrowded schedule.
Ferrero will play Switzerland's Stanislas Wawrinka, who knocked out Andy Murray, in the last eight.
The defeat is Nadal's first on clay since he lost 2-6 6-2 6-0 to Roger Federer in the final at Hamburg last year and his first ever defeat in Rome.
I congratulate Juan Carlos, but today for sure was not my best tennis
Nadal had a chance to break Ferrero with the score at 4-4 in the first set but lost four points in a row.
He survived a set point in the next game but was eventually broken at 6-5 with Ferrero hitting a stunning backhand winner.
The world number two immediately called for treatment on his blistered foot and did so again with the score at 4-1 to Ferrero in the second set.
Nadal had lost his serve at 2-1 and, with his movement restricted, was unable to force his way back into the contest.
After his defeat Nadal explained that his participation in the match had been in doubt.
"Juan Carlos is a very tough opponent, but certainly if you're not 100% at a Masters Series event it is very tough," said Nadal.
"Today when I woke up, I said it was impossible to play. I spoke to the doctor today and yesterday and they put special protection on it and cream, but it was still tough.
"I congratulate Juan Carlos, but for sure that was not my best tennis."
Ferrero said: "I had never won a set against him on clay so I'm pretty happy of course.
"It's a very important win for me and I feel special right now. I haven't had this special feeling for a long time.
"I was so strong mentally today. I had things very clear in my mind."
With the addition to this year's calendar of the Olympic Games in August, the clay-court schedule has been squeezed and left Nadal playing consecutive weeks in Monte Carlo, Barcelona, Rome and next week in Hamburg.
"The calendar is impossible," he said. "Last week in Barcelona I said it was going to be impossible for me to play well four weeks in a row," he said.
"I'll try to have some rest at home and go to Hamburg with my best feelings."
Elsewhere, Novak Djokovic, the Australian Open champion, eased through with a 6-4 6-0 win over Belgium's Steve Darcis.
The Serb then confirmed that he is getting back to full fitness after illness forced him to withdraw from his Monte Carlo semi-final against Roger Federer two weeks ago.
"I finished with my medication yesterday," said Djokovic. "So hopefully now I'll get better in the future."
And asked about his good start to the year, he added: "The Australian Open changed my life.
"Now I want to become number one this year."
But seeds David Nalbandian and David Ferrer were both knocked out.
Argentine Nalbandian, the seventh seed, lost an exciting match against Spain's Nicolas Almagro 6-4 7-5.
Spanish fifth seed Ferrer won the first set against Radek Stepanek of the Czech Republic but ended up losing 4-6 6-2 6-1.