Rafael Nadal of Spain poses with the Ion Tiriac trophy after his victory over Kei Nishikori of Japan in their men's singles final match at the Madrid Open tennis tournament May 11, 2014. Nadal won a second successive Madrid Open title when Nishikori was forced to retire in the third set of Sunday's final with an apparent back injury. [Photo/Agencies]

MADRID - Rafael Nadal finally won another clay-court title, although not in a manner that alleviated any concerns about his recent form on his favorite surface.

The top-ranked Spaniard won his fourth Madrid Open on Sunday after Kei Nishikori was forced to withdraw with a hip injury when trailing 2-6, 6-4, 3-0 in the final.

And Nadal, who is coming off to uncharacteristic quarterfinal losses on clay, said his mental strength is still lacking just two weeks ahead of the French Open.

"I was blocked. There were some moments where, I don't know, I couldn't find myself," said Nadal, whose victory guarantees he will stay No 1 in the rankings heading into Roland Garros. "It wasn't that I didn't want to play or I was missing intensity, I was just mentally blocked."

Earlier, Maria Sharapova bounced back from a poor start to defeat Simona Halep 1-6, 6-2, 6-3 to win the women's title.

Nadal became the first repeat winner in Madrid but it was only his second clay-court title of the year, and third overall in 2014.

Nishikori's aggressive start allowed him to jump out to a 5-1 lead as his powerful groundstrokes overwhelmed Nadal, who struggled to find an answer against his Japanese opponent.

"We don't deserve the victory, (Nishikori) deserves it, he played better than us the whole time," Nadal's uncle and coach Toni Nadal told Antenna 3 TV. "We had a lot of luck today. We didn't really come back, he was hurt."