* Nishikori sustains back injury when closing on victory
* World number one Nadal claims 63rd career title
* Doubts remain over Spaniard's form before French Open (Adds details, quotes)
MADRID, May 11 (Reuters) - Rafa Nadal won an unconvincing second straight Madrid Open title after Kei Nishikori sustained a back injury when closing on victory in the second set of Sunday's final and was forced to retire early in the third.
Japan's Nishikori, the 10th seed and one of the game's rising talents, made a blistering start on Nadal's favoured clay to win the first set 6-2 and was a break up in the second and serving at 4-3 when disaster struck.
After a long rally with the score at 15-30, the Japanese pulled up clutching his back and Nadal went on to win the game and the set 6-4 to force a decider.
In a tremendously disappointing finish to an entertaining encounter, Nishikori could barely walk to the net to shake hands after Nadal had raced into a 3-0 lead in the third.
"It was my hip and actually everywhere after the second set, my legs were hurting too much," Nishikori, who came through a gruelling three-set semi-final against David Ferrer, told a news conference.
"It was very sad, especially as I was winning, playing almost the best tennis in my life," added the 24-year-old, contesting his first Masters 1000 final and fresh from his debut title on clay in Barcelona last month.
"But there is some, you know, good side too. There is a lot of confidence I get from this tournament."
The premature end to Sunday's showpiece left the jury out on Nadal's form ahead of his bid for a record-extending ninth French Open singles title when the clay grand slam starts at the end of the month.
His latest triumph, his fourth Madrid crown, was the 27-year-old Majorcan's 63rd career title and 44th on clay, two short of Guillermo Vilas's all-time record of 46. He has 27 Masters titles, also a record.
However, he has struggled on the red dust this season, losing in the quarter-finals in Monte Carlo and Barcelona and there is a sense that his days as the king of Roland Garros may be numbered.
Certainly the way Nishikori dictated play in the early stages will have set alarm bells ringing but Nadal still has this week's Rome Masters to fine-tune his game before heading to Paris.
"There was some moments where, I don't know, I couldn't find myself," Nadal told a news conference.
"It wasn't that I didn't want to play or I was missing intensity, I was just mentally blocked," he added.
"When you're blocked, you just need a spark to go out there and compete again.
"I was just fighting because if I had the chance I was going to be there to take it."
Asked about his prospects for the French Open, Nadal added: "Well, much better, of course. Whenever you win in sport, well, it's something basic. It's a vital part of sport.
"Even though I think that today I did a not so good first set, I think I did a great tournament. This is the reality of the situation." (Reporting by Iain Rogers, editing by Ed Osmond)