NEW YORK (AFP) - World number one Rafael Nadal battled his way into the US Open quarter-finals Monday, outlasting 55th-ranked Sam Querrey 6-2, 5-7, 7-6 (7/2), 6-3 to match his best career showing at the Grand Slam event. The reigning Wimbledon, French Open and Olympic champion is trying to become only the fourth man to win three Slam singles crowns in a row, joining Pete Sampras, Rod Laver and the man he dethroned atop the rankings, Roger Federer. "Very tough," Nadal said. "I'm very happy for the victory. Now I'm in my best round at the US Open and I hope to play better in the next round." But Nadal struggled in the face of an epic effort from lanky US 20-year-old Querrey, who nearly made his fourth-round Slam debut a shocker for the ages and cast major doubt upon whether or not Nadal has what it takes to win a US Open. "He had to earn that third set and fourth set," Querrey said. "It's nice to know he actually had to go out there and fight for it rather than me handing it to him." The 22-year-old Spanish left-hander bids for his first US Open semi-final against 35th-ranked American Mardy Fish, who matched his best Slam run from last year's Australian Open by defeating France's Gael Monfils 7-5, 6-2, 6-2. "He's a dangerous player. He's playing very well," Nadal said. "He's playing with big confidence and I have to play very well if I want to win." The other quarter-final on Nadal's side of the draw will send British sixth seed Andy Murray against the hottest player on tour, Argentina's 19-year-old Juan Martin Del Potro. The South American won his 23rd match in a row, beating Japan's 18-year-old Kei Nishikori 6-3, 6-4, 6-3 to advance, while Scotsman Murray ousted a friend, Swiss 10th seed Stanislas Wawrinka, 6-1, 6-3, 6-3. "It's always tough playing against one of your friends," Murray said. "But this is the quarter-finals of my favorite tournament and I had to put the friendship aside." Murray, who reached his first Slam quarter-final this year at Wimbledon, assured a rise to fifth in the world with the triumph. He beat Del Potro in May on Rome clay in their only prior meeting when his foe retired in the third set. Querrey fired 20 aces and 52 winners at Nadal but made 50 unforced errors to fall short after three hours and 13 minutes at Arthur Ashe Stadium. "At the start I was a little nervous but after an hour I was hiting my shots pretty good," Querrey said. "I was just taking my forehand and trying to rip it as hard as I could. He's just too good." Nadal broke Querrey in the third and seventh games to take the first set in 31 minutes, but in the second, Querrey answered a Nadal break in the seventh game by breaking at love with Nadal serving for the set to equalize at 5-5.Querrey held and ripped a forehand cross-court winner to grab two break points on Nadal in the 12th game. Nadal netted a forehand on the next point and the fight was on."It was very difficult, especially after my big mistake in the second set," Nadal said. "I was playing normal. Nothing special but nothing bad." Nadal broke to open the third set but Querrey answered in the eighth game, breaking back on Nadal's fifth double fault to set up the pivotal tie-breaker. Nadal jumped ahead 3-1 in the tie-break when Querrey sent a forehand wide, then won the final four points, three on errant shots by the American, to take the set, jumping for joy when a Querrey backhand went long to seal the set. "I served very bad in the second set but the important thing was I played my best in the tie-breaker," Nadal said. Querrey, who won his first ATP title last March at Las Vegas, was down 4-2 when he forced seven break chances in the seventh game. But Nadal yanked each back from the brink, tension growing with every rescue until two errant Querrey forehands allowed Nadal to hold for 5-2 and he held again to end it. "If I could have battled through that game and got back on serve, you never know," Querrey said. "The longer the match went on the more free I was on my forehand. It was feeling good, especially in the fourth set." If Fish has anything to say about it, another battle awaits Nadal. Fish, 26, was a runner-up last week at New Haven and beat then-number one Federer in March at Indian Wells. He said he was "absolutely" confident he could hurt Nadal. "Confidence is a big thing. It comes back to wanting desperately to do well and do anything I can to win," Fish said. "A guy with my style of play is someone he doesn't want to see. "Rafa is playing so well and so solid from the baseline and moves so well. You've got to be able to finish points quickly," Fish said. "He can finish with anybody. He wants to run guys down. I don't intend to let him do that. "I'm going to come in - not necessarily kamakaze-type tennis, but I'm going to try to keep the points as short as possible."