World number two Rafael Nadal captured his sixth title of the year on Sunday, overcoming a stiff challenge from unseeded Stanislas Wawrinka to win 6-4 7-5 in the final of the Stuttgart Cup.

The French Open champion, competing in his 16th straight clay-court final, was made to work hard by the Swiss world number 50 and had to come back from 5-2 down in the second set to take victory in just over two hours.

"I played a really good tournament this week and that has given me a lift after losing at Wimbledon," Nadal said in a television interview.

It was his second win at the Stuttgart Cup after his 2005 triumph and he improves his 2007 win-loss record to 56-8, with 31-1 on clay.

The 21-year-old Majorcan had problems with his right knee earlier in the tournament which almost forced his withdrawal. He was again wearing strapping on Sunday but showed no obvious signs of discomfort during the match.

"It was the same problem I had at Wimbledon," Nadal said. "But once it was taped up the pain subsided a little and I am very happy I decided to carry on."

Wawrinka, who won his only ATP title last year, was out for three months earlier in 2007 with a knee injury but pushed Nadal harder than any of his opponents at the Weissenhof this week, breaking the top seed three times.

The Swiss produced a string of stunning drives and showed great touch and athleticism in successfully chasing down several of Nadal's trademark drop shots but folded when serving for the second set at 5-3.

"I lost the match today but I played really well and it's been an unbelievable week for me," Wawrinka said.

His compatriot, the world number one and five-times Wimbledon champion Roger Federer, had been sending him SMS messages of support throughout the week, Wawrinka said.

"But unfortunately he didn't tell me how to beat Rafa," he added, before spraying his opponent with champagne.

Nadal has now won 93 of his 94 matches on clay since April 2005, his only defeat coming against Federer in the final of the Hamburg Masters in May.