Kei Nishikori of Japan returns a ball to Rafael Nadal of Spain during the Madrid Open final tennis match 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]

Nishikori broke Nadal again to start the second set to leave the Magic Box center crowd, which included Queen Sofia, dumbfounded to find Nadal struggling so badly.

Nishikori first experienced discomfort while leading 4-2 and Nadal took advantage to break back. Nishikori soon needed a medical timeout for treatment on his back and hip, and his energy then looked sapped as he hit a shot long to concede the set.

Nishikori, who withdrew from next week's tournament in Rome to recover in time for the French Open, visibly struggled in the third before eventually retiring.

"It was very sad, especially (since) I was winning, playing almost best tennis in my life," said Nishikori, who will become the first Japanese player in the top-10 when the new rankings are released. "It was very tough mentally for me. But I get a lot of confidence from this tournament.

"You never know what happens today if I'm not hurt. I may have chance to win."

Sharapova, who lost to Serena Williams in last year's final, crumbled in the opening set when she held her serve just once.

But Halep's serve dipped in the second set, and Sharapova started hitting pinpoint ground strokes that kept the fifth-ranked Romanian running. The ninth-ranked Russian converted both her break points in the set to even the match.

Sharapova then pulled away in the deciding set with an early break to follow up her triumph in Stuttgart with a maiden title in Madrid.

Since the 2011 French Open, Sharapova has a 47-3 record on clay - with all three defeats coming to Williams. The top-ranked American was the two-time defending champion in Madrid but withdrew with a leg injury on Friday.

"I don't know how I pulled it off," Sharapova said after winning her 32nd career title. "I came close last year, and I didn't have a great first set today, but I knew it wasn't over until the last point was played."

Sharapova planned to continue her center court celebration on the plane ride to the Rome Masters.

"They better have some champagne for me," Sharapova said. "I need a drink."