Tennis: Nadal knees hold up as he regains No.2 spot with victory
Nadal's victory, combined with Murray's shock defeat to Croatian Marian Cilic, having reached the final 12 months ago, meant the Spaniard regained the No.2 spot but that was just an added bonus as he kept alive his dream of completing a full sweep of the major tournaments at the age of 23.
Nadal came from a set down to France's 13th seed to win 6-7 (3-7), 6-3, 6-1, 6-3 and set up a last-eight meeting with Chile's Fernando Gonzalez, the 11th seed, who beat seventh seed Jo-Wilfried Tsonga of France.
"I'm very happy now after the last two matches I played," Nadal said having improved on previous round performances against Richard Gasquet and Nicolas Kiefer.
"I had a few problems in those but now I'm in the quarter-finals for the third time in my career here and I'm really, really happy. I'm here just to play my best and try to enjoy my comeback. For me to come back here and play in the quarter-finals is very big, a big result for me."
The win also dispelled fears Nadal was playing while hurt having aggravated an abdominal injury during his match in the previous round against Kiefer, while the US Open marked just his third tournament back after a two-month lay-off with tendinitis in both his knees. "I'm improving every day after two months of stopping (playing]," Nadal said.
"It's not easy coming back but I'm very happy with quarter-finals in Montreal and semi-finals in Cincinnati and quarter-finals here is much better than I expected.
"The knees are very good, so I'm very happy."
Monfils certainly brought the best out of Nadal with his athleticism and willingness to fight for every point.
"He's unbelievable. Sometimes you think you've won the point and the ball is coming back. He's a very good player and I had to play really well."
Of his next opponent, Gonzalez, Nadal said: "I saw him today playing Tsonga while I was in the locker room and he did very well, unbelievable forehands.
"If I don't play my very best it's going to be impossible."
Cilic, meanwhile, will face sixth seed Juan Martin del Potro, who cruised into the last eight with a 6-3, 6-3, 6-3 win over Spain's 24th seed Juan Carlos Ferrero.
Cilic will be making his first appearance in a major quarter-final and del Potro said: "He has everything to be a good player but Murray is No.2, winning everywhere.
"So maybe everybody is surprised but Cilic has very good weapons. If he beat Murray he's confident and it will be a very tough one for me."