Federer out of focus ahead of US Open

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.

Federer, 31, who has been occasionally troubled by back spasms in a relatively injury-free career, said he has been dealing with them for a few weeks now.

He hinted he will be making a late decision on whether or not to play the Montreal Masters, a key US Open warm-up event, which starts on Aug 5.

"I've had serious problems with the back; I had to get some anti-inflammatories last week in Hamburg due to the pain," Federer said after Thursday's loss, his third of the summer against an opponent ranked outside the top 50.

Federer is currently ranked fifth in the world, his lowest level in a decade after going out in the Wimbledon second round to Sergiy Stakhovsky and then losing last week in the Hamburg semifinals to Argentine qualifier Federico Delbonis.

Federer had been hoping to both build confidence as well as test out a new larger racquet. Instead, he must now work on his back and hope for the best.

"I will have to do a lot of exercises and see how it all feels. My main priority now is to fix my back. I would love to be able to train at 100 percent," said the former world No 1.

"I'll have to see if the rehab is enough to let me play in Montreal. If it is, I'll go; if not, then it gives me another week."

Federer said that due to his physical problems it's difficult for him to tell where he is with his new racket anyway.

"A change like this is very important but I honestly can't tell right now due to my low level. It's hard to analyze anything.

"I still don't have even enough information to try and explain. I have no clue where I am right now with the racket change."