Roger Federer admitted that a back injury contributed to his latest shock defeat, a 6-3, 6-4 loss to Germany's world number 55 Daniel Brands in the Swiss Open second round.
Amid suspicions of a recurrence of the problem which has affected him in the past, 31-year-old Federer confirmed that the injury has indeed returned.
"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 US Open build-up event, 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.
"My back felt OK enough, but not enough to play at 100 percent. I knew coming into the match that it would be a tough one. I didn't decide to play until after the warm-up, that's what a close call it was.
"I played a decent enough match but I could not get away with the win."
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, he 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.
Federer had made a celebrated return to the small claycourt tournament at Gstaad where he got his start on the ATP Tour 15 years ago as a teenaged wildcard entry.
He lost in the first round in each of his first four appearances, from 1998 through 2001.
The fifth-ranked Swiss -- his lowest standing in a decade -- was competing in this elite alpine village at 800 metres nine years after last appearing in a title-winning performance.
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.
"I played at 100 per cent, I didn't have a big strategy, I just played point after point and tried to stay strong mentally."
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.
Serb third seed Janko Tipsarevic was knocked out in his opening match 6-2, 6-2 by Dutchman Robin Haase.
Tipsarevic has had a dreadful showing this season, slipping out of the top 10 over the last four months.
He began in January with a title in Chennai but has since managed only three quarter-finals.
The number 18 had no answer to Haase, playing the tournament for the first time and now standing 16-18 this season and now one victory away from a 100th career success.
Tipsarevic went down two breaks in the second set and managed briefly to stop the rot before eventually falling in 61 minutes with six double-faults and dropping service four times.
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 last week.