Last updated on .From the section Tennis

Roger Federer won the Australian Open January before adding his eighth Wimbledon title in July

US Open Venue: Flushing Meadows, New York Dates: 28 Aug-10 Sept BBC coverage: Live radio and text commentary on selected matches every day.

Roger Federer said he never felt "in a safe place" physically or mentally during a US Open campaign that ended with defeat by Juan Martin del Potro.

The Argentine 24th seed won their quarter-final 7-5 3-6 7-6 (10-8) 6-4 in Wednesday's night session.

Third seed Federer had struggled with a back injury in the build-up to New York and confirmed he never felt 100% fit.

"It was one of those matches where if I ran into a good guy I was going to lose, I felt," said the Swiss, 36.

"I don't want to say I was in a negative mindset, but I knew going in that I'm not in a safe place. It might have depended too much on my opponent - and I don't like that feeling.

"I had it throughout the tournament, and I just felt that way every single match I went into."

Federer has enjoyed a stellar year, ending his five-year wait for a Grand Slam title at the Australian Open and adding an eighth Wimbledon victory in July.

The 19-time major champion added: "I didn't have that feeling at Wimbledon or at the Australian Open, and that's why rightfully so I'm out of this tournament.

"I wasn't good enough in my mind, in my body and in my game to overcome these three pillars."

Having skipped the French Open, Federer arrived at Flushing Meadows unbeaten in Grand Slam matches and with just three defeats all year.

However, the back injury he picked up in Montreal last month disrupted his preparation, and two five-set matches in the first two rounds suggested he was not at his best.

The defeat by 28-year-old Del Potro ended Federer's hopes, for now at least, of winning a 20th Grand Slam title and taking the number one ranking from Spaniard Rafael Nadal.

"It should hurt, and it does, rightfully so," said Federer. "But I think my perspective at this age, and with the season that I have had, it's easier to grasp faster, so I'll be fine quickly."

The Swiss had his chances against Del Potro, notably four set points in a dramatic third set tie-break that could have swung the match in his favour.

"I'm OK with it, and I tried until the very end," he said.

"Smashing certain stuff in the net that I normally wouldn't, smashing forehand volleys into the back fence, I mean, that stuff sucked. Honestly, it was terrible.

"Juan Martin did well. He served well, had some big shots when he needed to. That was the part that he did so much better tonight - and that's why he deserves to win.

"I feel I have no place in the semis and he will have a better chance to beat Rafa, to be honest."

The result not only brings his Grand Slam season to an end but it denies the Flushing Meadows crowd a first meeting between Federer and 15-time major champion Nadal.

It is the sixth time in 10 years that they have missed out by one match on seeing the great rivalry play out in New York.

"I didn't even think about it as I lost that match," said Federer.

"I'm dealing with just trying to understand what happened and just to overcome this in the next few hours, days, weeks, whatever it is. I'll be fine.

"Of course it is a pity, but Juan Martin deserves it more."

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Analysis

BBC tennis correspondent Russell Fuller

Federer says he hasn't felt well enough in either body or mind over the past fortnight, his confidence affected by the back injury which hindered his preparation.

He looked an unlikely champion when labouring to five-set victories in the opening two rounds and when faced by a competitor like Del Potro he was made to look very uncomfortable and very uncertain.

There were some spectacular misses by Federer, who too often presented himself at the net and invited Del Potro to pass him with his fearsome forehand. The Argentine took Federer up on his offer repeatedly, and served consistently well throughout the match.

And so Federer's attempt to win three Grand Slams in a year for the first time in a decade has ended at the quarter-final stage.

The ever popular Del Potro, who has had wrist surgery four times since winning the title here in 2009, is one win away from a return to the final of his favourite Grand Slam.