(CNN) -- When Novak Djokovic ended Rafael Nadal's eight-year reign at the Monte Carlo Masters in April, it marked the perfect start to the world No. 1's clay-court season.
But Djokovic's ensuing performances on clay haven't been anywhere near as impressive.
After losing his opening match to Grigor Dimitrov at the Madrid Masters last week, Djokovic fell in the quarterfinals of the Rome Masters on Friday against a player who he previously held a 13-1 record against.
Tomas Berdych rallied from a set down to beat Djokovic 2-6, 7-5, 6-4, denting the Serb's preparations for the French Open, which begins May 26. Djokovic couldn't serve out the match at 5-3 in the second set.
"I lost the concentration," Djokovic told reporters. "I started playing more defensively. I played a bad game at 5-3 and the match turned around. He started playing more confidently, stepping into the court and taking advantage of his opportunities.
"It's a very strange situation. I played very well and was two points from winning. Then the entire match changed and I was a different player."
Djokovic's defeat opened the door for Nadal and the defending champion continued his mastery over fellow Spaniard David Ferrer.
Nadal prevailed 6-4, 4-6, 6-2 to improve to 16-1 against Ferrer on clay -- Ferrer won their first match in 2004 -- and set up a showdown with Berdych in a repeat of the 2010 Wimbledon final, which Nadal won.
If Nadal claims the title in Rome, he will leapfrog Ferrer to No. 4 in the rankings in time for the French Open seedings.
Frenchman Benoit Paire beat Marcel Granollers 6-1, 6-0 to advance to his first Masters semifinal and will face Roger Federer in the last four.
Meanwhile in the women's draw, a repeat of the Madrid final between the world's top two failed to materialize when Maria Sharapova withdrew from her quarterfinal with home favorite Sara Errani due to illness.
Sharapova, the reigning French Open champion, lost to Serena Williams in straight sets in the Spanish capital.
"I was already a bit off in Madrid and played through it and it came back last night," Sharapova said. "And I got sick again and it's not smart to compete again two or three rounds to win the tournament and with Paris starting next week.
"The doctor thinks I didn't take it easy last week and if I don't take care of it now, then it might worsen. It is one of those things, and the body is not ready and with a bigger goal around the corner it's important to make the right and smart decisions. I have a lot of clay matches and so this is not lacking before the French Open."
Williams crushed Carla Suarez Navarro in under an hour to extend her winning streak to a personal best 22 matches and next meets Romanian qualifier Simona Halep in the semifinals.
Errani, runner-up to Sharapova at Roland Garros, confronts Victoria Azarenka in the other semifinal.