World number three Rafael Nadal put his injury worries behind him to beat France's Gael Monfils and reach the quarter-finals of the US Open.

The Spaniard, who is on his way back from a serious knee injury, has also had an abdominal problem but showed no sign of it in Tuesday's night match.

Nadal showed great reserves of strength to win 6-7 (3-7) 6-3 6-1 6-3 and set up a match with Fernando Gonzalez.

Juan Martin del Potro and Marin Cilic also won to set up a quarter-final tie.

In the final match on Arthur Ashe Stadium, Monfils edged a high-quality first set in the tie-break after failing to serve out at 5-3, with his huge serving and heavy forehands taking him through.

But after some exuberant celebrations, the Frenchman soon found that Nadal was going nowhere and had retained his incredible stamina.

After the pair swapped breaks midway through the second set, Nadal broke again in a breathless eighth game of punishing rallies that left his opponent exhausted.

Monfils could only win four more games as Nadal hit the kind of form that has taken him to six Grand Slam titles, and suggests he could complete the set with his first US Open victory.

And afterwards he described the conditon of his knees as "very good" before adding: "I'm very happy now after the last two matches I played.

"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, so very happy."

After the match had finished, a spectator ran on court and broke through security to kiss Nadal as he sat on his chair changing his shirt, but the Spaniard made light of the incident.

"For me it wasn't a problem. The guy was really nice, he was a great fan, he said I love you," said the player.

This is the second security incident at a grand slam tournament this year, after a fan ran onto the court during the French Open final and put a hat on world number one Roger Federer.

Chile's Gonzalez, seeded 11th, beat seventh seed Jo-Wilfried Tsonga of France 3-6 6-3 7-6 (7-3) 6-4 in a pulsating match on Louis Armstrong Stadium.

Gonzalez gave his racquet to a woman in the crowd at one point as he struggled to control his temper early on, but the 29-year-old stormed back and took control in the third set tie-break.

Trailing 5-4 in the fourth set, the pressure became too much for Tsonga and he double-faulted twice as he handed over the game and the match.

"In my first match here I didn't feel like I really wanted it," said Gonzalez. "Today I feel much better, I play really good tennis again. Hopefully I can continue this week."

Sixth seed Del Potro proved too strong for Juan Carlos Ferrero as he moved into the last eight with a 6-3 6-3 6-3 win.

The Argentine's big serve was the foundation of his victory as he served 22 aces on his way to victory, without ever being broken.

Del Potro lost to Murray in the quarter-finals last year but has since risen to sixth in the rankings and narrowly lost to Federer in five sets in the French Open semi-finals.

"Being in the quarters, it's important," said Del Potro. "But it's a big difference past the quarters to the semis and then through to the finals.

"I was so close in the French Open to reach the final and to beat Roger. Here I have another chance to be better and I will try to go the semis. If I lose I still had a good tournament."

The 20-year-old's improved serve has been a major factor in his success, along with his power off the ground, and he made 44 winners as he dominated Ferrero.

"When you play somebody like this, you feel a lot of pressure with your service games," Ferrero said afterwards.

"I think he's playing a level to win a Grand Slam. I think he can play against anybody here. He has to play against Cilic right now, I think this is a chance for him to get into the semis."

Croatia's Cilic, the 16th seed, beat Britain's Andy Murray 7-5 6-2 6-2 in the day session on Arthur Ashe Stadium to reach his first Grand Slam quarter-final.

"Now that I don't have this blockade in my head, I can focus on winning other matches," said the 20-year-old. "I'm feeling tremendously happy. It's the biggest result for me so far."

In the other half of the draw, defending champion Roger Federer, Robin Soderling, Novak Djokovic and Fernando Verdasco are already through to the last eight.

From BBC Sport

Kim Clijsters continued her remarkable run at the US Open, beating Li Na 6-2 6-4 to set up a semi-final against defending champion Serena Williams.

The 26-year-old Belgian, who has come out of retirement to play in her first Grand Slam since the 2007 Australian Open, made a confident start.

Li briefly rallied in the second set but a series of errors sealed her fate.

Clijsters will next play Williams after the American beat Italy's Flavia Pennetta 6-4 6-3 in the night session.

Pennetta...

Reigning champion Roger Federer booked a US Open quarter-final clash against Robin Soderling with a 7-5 6-2 6-2 win over Spaniard Tommy Robredo.

The world number one broke Robredo in the 11th game to clinch a tight first set before breaking twice more in a far more comfortable second.

An aggressive Federer wrapped up victory in the third set with an ace.

Novak Djokovic remains on course for a semi-final against Federer after beating Radek Stepanek 6-1 6-3 6-3.

Djokovic, seeded fourth,...

American Melanie Oudin continued her remarkable run at the US Open with a 1-6 7-6 (7-2) 6-3 win over Nadia Petrova to reach the quarter-finals.

It seemed that luck had run out for the 17-year-old as she was overpowered in the first set by the Russian 13th seed.

But back came Oudin to win the next on a tie-break and she then broke serve twice in a tense final set to record another famous victory.

Oudin will play Danish ninth seed Caroline Wozniacki in the last eight.

Wozniacki, 19, bea...

World number three Rafael Nadal played through pain to beat fellow Spaniard Nicolas Almagro 7-5 6-4 6-4 and stay in the hunt for his first US Open title.

Nadal, who missed this year's Wimbledon through tendinitis in both knees, was hampered in the later stages by a midriff injury and required treatment.

The 23-year-old looked short of his best but Almagro repeatedly lost the initiative after taking the upper hand.

Juan Martin del Potro and Jo-Wilfried Tsonga both won in straight sets.

Kim Clijsters claimed her biggest win since coming out of retirement as she defeated third seed Venus Williams 6-0 0-6 6-4 in the US Open fourth round.

The former world number one, in her first Grand Slam for two years, breezed through the opening set with a flurry of powerful winners.

Williams regrouped and came blazing back, but was undone as the Belgian clung to a solitary break in the third.

Serena Williams advanced with a 6-2 6-0 demolition of Daniela Hantuchova.

Clijsters and Ve...

Defending champion Roger Federer moved into round four of the US Open with victory over Lleyton Hewitt but Andy Roddick was knocked out by John Isner.

Top seed Federer came from behind to beat Hewitt 4-6 6-3 7-5 6-4 and will next play 14th seed Tommy Robredo.

But Roddick, seeded fifth, was stunned 7-6 (7-3) 6-3 3-6 5-7 7-6 (7-5) by unseeded fellow American Isner in a thriller lasting just over four hours.

Fourth seed Novak Djokovic battled to a four-set win against Jesse Witten.

Ferna...

World number one Dinara Safina was knocked out of the US Open after an error-strewn display led to a 6-4 2-6 7-6 (7-5) defeat by Petra Kvitova.

Safina made 39 unforced errors and served nine double faults as the unseeded Kvitova, 19, won in two hours and 34 minutes.

Earlier, unseeded American Melanie Oudin beat former world number one Maria Sharapova 3-6 6-4 7-5.

But sixth seed Svetlana Kuznetsova won through, beating Shahar Peer 7-5 6-1.

It was the kind of result Kuznetsova's fellow ...

World number three Rafael Nadal was given a stern test by Nicolas Kiefer before coming through in four sets to reach round three of the US Open.

The Spaniard dominated the first set but a number of unforced errors allowed the veteran back in before he regained control to win 6-0 3-6 6-3 6-4.

The 23-year-old will next face 32nd seed and compatriot Nicolas Almagro.

Elsewhere, sixth seed Juan Martin Del Potro and seventh seed Jo-Wilfred Tsonga eased through in straight sets.

World number...

The Williams sisters had to utilise all their resolve and battling prowess to progress into the US Open fourth round.

Defending champion Serena snatched the first set against Maria Jose Martinez Sanchez and then survived a second-set fightback before prevailing 6-3 7-5.

Third seed Venus produced an erratic display against Magdalena Rybarikova and, after struggling on serve throughout, eventually won 6-2 7-5.

Serena now faces Daniela Hantuchova, with Venus up against Kim Clijsters.

Num...