Injured Roger Federer's worrying slump continued on Thursday when he was knocked out of the Swiss Open by world number 55 Daniel Brands in another shattering blow just a month out from the US Open.
The German spoiled former world number one Federer's return to the tournament after nine years with a shock 6-3 6-4 second-round upset.
Federer revealed he's been struggling with back pain in recent weeks, severely limiting his play.
He confirmed suspicions it was a recurrence of the problem that affected him in the past.
"I've had serious problems with the back, I had to get some anti-inflammatories last week in Hamburg due to the back pain," he said, adding that his first build-up for the season's final Grand Slam event of the year in New York, the Montreal Masters from August 5, could now be in doubt.
"It was so tough to play and move out there today. I'll just have to take treatment and see how it all goes."
World number five Federer has won only the Halle title this season after claiming his 17th Grand Slam title at Wimbledon in 2012.
Last week, the 31-year-old was stunned by Argentine qualifier Federico Delbonis, the world number 114, in the Hamburg semi-finals.
That defeat came just a month after Federer was knocked out of Wimbledon by Ukraine's world number 116 Sergiy Stakhovsky in the second round.
That brought to an end a run of 36 consecutive Grand Slam quarter-final appearances for the Swiss.
Brands came good on the threat he posed in the Hamburg first round eight days ago when he lost in three sets to Federer.
This time, the German got the win despite Federer saving two match points, the first with an ace on his own serve in the ninth game of the second set.
Brands completed the fourth win of his career over a top 10 player a game later after Federer, who struck more than 20 unforced errors, saved another match point before falling to a Brand service winner.
"This is a really special win," said Brands.
The German will next face Victor Hanescu, who advanced to the quarter-finals when Spanish eighth seed Roberto Bautista Agut retired with back pain trailing 7-6 (7-2) 3-2.
Serbian third seed Janko Tipsarevic was knocked out in his opening match 6-2 6-2 by Dutchman Robin Haase.
The 2011 Gstaad winner, Marcel Granollers, moved into an ATP quarter-final for the third time this season with a 6-2 7-5 second-round elimination of Delbonis, the runner-up in Hamburg.
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