London: Rafael Nadal looked ahead Wednesday to the latest chapter in his "unique" rivalry with Roger Federer at Wimbledon but regretted that the two heavyweights may never slug it out at the US Open.

Nadal and Federer will meet for the 40th time on Friday in the semi-finals at Wimbledon, in what will be the pair's fourth meeting at the All England Club. Federer won the 2006 and 2007 finals before Nadal claimed the 2008 title in a five-set epic. At Roland Garros, the pair have clashed six times while they have met on four occasions at the Australian Open. But when it comes to the season's final Grand Slam in New York, the two have never managed to face off on the 24,000-capacity Arthur Ashe Stadium, the sport's largest arena.