Novak Djokovic reached the US Open quarter-finals with a 7-5 7-6 (7-2) 6-7 (6-8) 6-1 win over Juan Monaco.
The Serb, seeded third, was not at his best throughout the match but broke in game 12 to seal the first set and took the second on a tie-break.
But he let an advantage slip in the third and missed a match point in the breaker before Monaco edged it.
An early break in the fourth set proved decisive, though, as Djokovic raced through to seal victory.
Second seed Rafael Nadal was taking on fellow Spaniard and 15th seed David Ferrer in Tuesday's final night match on Arthur Ashe Stadium.
"I had to fight hard for every point against him (Monaco) and I can say that I am pretty happy with my performance," said Djokovic.
"I played my best tennis at the most important moments and I served really well. I had soreness in my back and had to call the trainer.
"I feel like I'm 20 years old the way I'm playing"
"Then I had a little pain in my stomach and I felt out of energy also. I had some pills and vitamins so after that I felt pretty good.
"It's good that I have a day off and can recover."
He goes on to face Carlos Moya, who returned to the last eight for the first time in nine years with a 7-5 6-2 6-7 (5-7) 6-4 win over Ernests Gulbis on Arthur Ashe Stadium.
Moya and Gulbis, 19, met in a match which pitted the oldest man left in the draw against the youngest - and it was 31-year-old Moya's experience which told in the end.
The Spaniard used his groundstrokes to get the better of Gulbis in the opener, breaking in the 11th game, and he immediately broke in the second.
Each player had breaks in the third and Gulbis eventually took the set on the breaker, but Moya converted the only break points of the fourth, lashing down a backhand volley on his second match point.
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"I feel like I'm 20 years old the way I'm playing," said Moya. "It's great.
"I didn't expect to be in the quarters, but it's a great feeling to be here."
Juan Ignacio Chela saw off Stanislas Wawrinka 4-6 6-2 7-6 (8-6) 1-6 6-4 in Tuesday's other early fourth-round encounter.
Chela surrendered the opening set to Wawrinka when he went down 0-40 in the 10th game but the Argentine controlled the second to level the match.
The 20th seed edged the tie-break to win a scrappy third set but faded in the fourth to let Wawrinka back in.
Chela got the decisive break at 4-3 in the decider, serving out after three hours and 41 minutes to set up a tie with Nadal or Ferrer.
"It is my first time in the quarters here and the second time in my career so this is great for me. It's been a very good year," said Chela.
"I'm tired now, but I have a day off and I will be OK for the semis.
~RS~q~RS~~RS~z~RS~22~RS~)