Rafael Nadal won his first European clay title of the season in unfortunate circumstances at the Mutua Madrid Open when Kei Nishikori retired through injury.

Having trailed by a set and a break, Nadal won seven straight games to lead 2-6 6-4 3-0 when Nishikori called it a day.

The Japanese player, who will break into the top 10 for the first time on Monday, struggled with a back spasm in an epic win over David Ferrer on Saturday and the problem returned at just the wrong moment.

Nishikori was playing in his first Masters series final and for a set and a half completely out-played an out-of-sorts Nadal.

Nishikori's aggressive approach and superb hitting had the clay-court king in all sorts of trouble and Nadal will know there remain very real questions about his form despite his victory.

Nadal had looked back to something like his best after quarter-final losses in Monte Carlo and Barcelona with four straight-sets win, and began the final with six successive points.

But Nishikori saved a break point and then exploited Nadal's horrible lack of rhythm by forging ahead 5-1.

Nadal saved a set point but Nishikori quickly closed it out and then broke serve again at the start of the second set.

Nadal had three chances to break in the next game, but missed them all, to the bemusement of himself and the Madrid crowd.

But Nishikori called the trainer to have his back massaged at 4-3 and it became clear in the next game that his movement was becoming compromised.

He saved two break points but not a third and, although he received further treatment, the writing was on the wall.