MADRID - Rafael Nadal won his fourth Madrid Masters title on Sunday when Kei Nishikori was forced to quit their final with a back injury having had the Spaniard on the back foot. Japanese 10th seed Nishikori had taken the first set 6-2 and led 4-2 in the second before the back pain which had plagued him all week forced him to retire. That allowed top seed Nadal to claim a 2-6, 6-4, 3-0 win for his 27th Masters title and 63rd career crown after one hour and 41 minutes on court."I'm sorry for him," said Nadal. "Of course when you have the dynamic that he's having, when you suffer something like that, it's really tough. I suffered a similar situation, nearly the same, in Australia this year. So I know what I'm talking about and how bitter it is, especially when you're playing an important match. All of us have a moment when we have to face it, and today it was his day."Nadal had lost quarter-finals in Monte Carlo and Barcelona -- the event that Nishikori won last month -- and needed every bit of his fighting spirit to go through. After taking a medical timeout after the ninth game of the second set after a 4-2 lead evaporated due to his condition, Nishikori, who moves into the top 10 on Monday for the first time, received treatment for his back on court. But his body was broken and the top Asian on the ATP tour found it almost impossible to play on.After making the attempt in a third set, he finally threw in the towel after three games. "I'm very sorry for what happened today," said Nishikori. "This was my first Masters 1000 final and I was really excited to play. But I was hurting already and trying to fight. But Rafa was too good today." Nishikori swept the opening set in a 35-minute surprise and began the second with a break of serve before his body began to betray him.Nadal now stands 7-0 over Nishikori and now heads to Rome next week to defend that trophy before he moves on to the French Open where he is an eight-time champion. "I'm where I am. I feel well physically," said Nadal. "I'm feeling better and better physically, better than a year ago. This is the most important thing."