Rafael Nadal won his fourth Madrid Open title on Sunday after Kei Nishikori was forced to withdraw with a back injury when trailing 2-6, 6-4, 3-0 in the final.

Nadal became the first repeat winner in Madrid, and recovered from two recent quarterfinal losses on clay to win his second title on the surface and third overall this year.

Nadal showed signs of improvement this week compared to his early exits in Monte Carlo and Barcelona, but still didn't look quite like the player who has won eight French Open titles.

"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."

After both players held serve to start, Nishikori jumped out to a 5-1 lead with powerful groundstrokes that overwhelmed Nadal, who couldn't find a way to counter his Japanese opponent.

"After the early breaks (Nadal) got tense and his opponent played at a great level," Toni Nadal said.

After being broken in his first service game in the second set, Nadal slowly clawed back into the match at the Magic Box although Nishikori was already in some discomfort when Nadal broke back for 4-4.

Nishikori needed a medical timeout to treat his sore lower back, and his energy looked sapped as he hit a shot long to concede the set.

Nishikori visibly struggled in the third before eventually retiring.

"I'm very sorry for what happened today for everybody," said Nishikori, who will become the first Japanese player in the top-10 when the new rankings are released. "Unfortunately I was hurting today and I was too hurt to fight."

Nadal's victory guarantees he will stay No. 1 heading into Roland Garros.