Defending champion Rafael Nadal dug deep to win the longest match of this year's US Open in a classic five-set quarter-final against Dominic Thiem - which finished at 2:03am local time.

World number one Nadal lost the opening set 6-0 after being outpowered by the Austrian ninth seed in New York.

However, the 32-year-old Spanish top seed recovered to lead two sets to one before Thiem levelled in a tie-break.

Nadal edged a tense fifth-set tie-break to win 0-6 6-4 7-5 6-7 (4-7) 7-6 (7-5).

The 17-time Grand Slam champion clinched victory when Thiem hit an overhead smash long on the first match point, the drama continuing until the last shot in an epic encounter lasting four hours and 49 minutes.

Nadal jumped over the net to console his Austrian opponent at the end, the pair embracing as those left inside Arthur Ashe Stadium rose to their feet to give them a thunderous ovation.

"I said to Dominic: 'I'm very sorry and keep going.' He has plenty of time to win. He will have his chances in the future without a doubt," Nadal said.

Asked about how got through the tense moments, Nadal added: "Suffering is the right word. It was a great battle."

He will play Argentine third seed Juan Martin del Potro, the 2009 champion, in Friday's semi-finals.

Thiem fails to gain revenge

Dominic Thiem also lost the French Open final to Nadal in June

Thiem was aiming to earn a measure of revenge over Nadal, who dismantled his game for a straightforward victory in the French Open final in June - the Austrian's only appearance in a Grand Slam showpiece.

Ultimately, he faced more disappointment against one of the sport's all-time great fighters.

Eventually Nadal's endurance came out on top in this match, which was his 17th at a Grand Slam to go past the four-hour mark.

In contrast, 25-year-old Thiem had never previously gone past the four-hour mark in his whole career.

Neither player outwardly showed signs of fatigue as they continued to trade blows deep into a high-octane match.

Stylish and powerful winners continued to flow from the racquets of both players until the end, despite the match entering an energy-sapping fifth hour in hot and humid conditions at Flushing Meadows.

Despite the drama, the 24,000-capacity Arthur Ashe Stadium was only about a third full when the match concluded under the lights, with many fans needing to leave early to get home.

Those who were left were treated to a tense final-set tie-break, which proved to be a fitting end to a great match.

"This match is going to be stuck in my mind forever - tennis is cruel sometimes," Thiem said.

"This match didn't deserve a loser, but there has to be one."

Nadal ultimately digests rare 'bagel'

Thiem has gained a reputation as one of the most powerful players on the men's tour, with Nadal identifying his thunderous groundstrokes as the danger before their last-eight tie.

And Thiem came out swinging against Nadal in a 24-minute opening set which left the packed crowd inside Ashe stunned.

Nadal had faced two gruelling matches against Karen Khachanov and Nikoloz Basilashvili coming into the quarter-final, and Thiem was keen to test his energy levels from the start.

Potent off both flanks, Thiem hit 13 winners and fired down five aces to comprehensively win the first set.

Nadal won just seven points as he was 'bagelled' for the first time at the US Open since a second-round defeat by American second seed Andy Roddick in 2004.

Thiem's comeback in vain

After that aggressive start, the question was whether Thiem could sustain his intensity over a longer period.

Thiem refused to entertain a different tactical approach and continued to go for broke - a risk and reward strategy which, although he did maintain, ultimately did not pay off in the cruellest of circumstances.

Nadal rediscovered his service game in the second set, levelling by going on to take two of three straight breaks at the end of the set as both players wobbled.

In the periods where Thiem's powerful groundstrokes were finding their target, Nadal was in trouble.

But more unforced errors were beginning to creep into Thiem's game - and at crucial times.

He showed both brilliance and naivety in a tight fourth set where he twice fought back from 40-15 down on serve and won just one of seven break points, eventually taking the tie-break to force a decider.

Thiem twice more had to recover from break-point deficits on his serve in the fifth, deservedly taking the match to the sudden-death finish it deserved.

Thiem won 171 points in the match, five more than Nadal, and hit 74 winners compared to 55 by the Spaniard.

"If we skip the first set it was an open match from beginning to the end," Thiem added.

"Then it ends up in the fifth set tie break and he made one more point than me."