Roger Federer's US Open preparations could be in danger after the 17-time Grand Slam winner was beaten in his opening match at the Swiss Open while playing with back pain.

The top seed, who was attending the small claycourt event for the first time since winning it nine years ago, was also dealing with the added stress of testing a new larger racket, coping with his physical limitations and facing a big hitter in Daniel Brands.

The German produced a 6-3, 6-4 win to make the quarterfinals after taking the Swiss star to three sets in Hamburg last week.