NEW YORK — Sam Querrey is so young that there isn't a trace of irony when he mentions growing up rooting for Andy Roddick and James Blake, two guys still very much on tour and still in their 20s.

Querrey is so new to this whole professional tennis gig that there is nothing but earnestness in his voice when he admits he's excited his matches are on TV.

And Querrey is so young, so new and so eager that he clearly means it when he plainly explains he's "looking forward" to facing No. 1 Rafael Nadal at the U.S. Open, a matchup the 20-year-old Californian set up Saturday by knocking off a seeded player for the second time in the tournament.

It's Querrey's first berth in the fourth round of a major championship.

"And, you know, it's great that it's the U.S. Open," Querrey said, words tumbling forth through a wide grin. "It's the one that, you know, if I had to pick one to win, it would be this one."

Heady talk for a kid who is ranked 55th, owns one career title and has lost more matches than he's won on tour — even after upsetting No. 14 Ivo Karlovic 7-6 (5), 7-6 (5), 6-2 in the third round Saturday.

Querrey added that to his victory over No. 22 Tomas Berdych in the first round. A far tougher test follows, of course: Wimbledon, French Open and Olympic champion Nadal, who won for the 41st time in his last 42 matches by brushing aside Viktor Troicki 6-4, 6-3, 6-0.

In other third-round action Saturday, French Open semifinalist Gael Monfils, seeded 32nd here, beat No. 7 David Nalbandian in straight sets, then showed off some club-worthy dance moves to mark a victory he called "gorgeous." Monfils now meets the winner of Saturday night's later match between two Americans, No. 9 Blake and Mardy Fish.

No. 6 Andy Murray of Britain came all the way back from a two-set deficit to defeat a fading Jurgen Melzer of Austria 6-7 (5), 4-6, 7-6 (5), 6-1, 6-4. Murray celebrated by pushing up his gray T-shirt's right sleeve and flexing his biceps — a signal to his fitness trainer and support team.

"When you're training and wondering why you do all the work and feeling sorry for yourself and what have you — you kind of push through and keep working," Murray said. "Then when you have moments like that on the court, you know, you feel like it's all worth it."

His next opponent is No. 10 Stanislas Wawrinka of Switzerland, who also dropped the first two sets before winning in five against Flavio Cipolla of Italy.

Cipolla refused to shake hands afterward, angry that Wawrinka questioned whether the Italian really was dealing with leg cramps during the match and that Wawrinka let out a yell of "Come on!" after one of Cipolla's 11 double-faults.

There were no such theatrics in women's play. Both Williams sisters — the only two past champions left in the field — won 6-2, 6-1 against seeded foes who, in theory at least, should have provided something more of a challenge.

No. 7 Venus Williams compiled a remarkable 32-4 edge in winners against No. 27 Alona Bondarenko of Ukraine. No. 4 Serena Williams — one of five women with a shot at moving up to No. 1 by tournament's end — was never troubled by No. 30 Ai Sugiyama of Japan.

With the retirements of Justine Henin and Kim Clijsters, a shoulder injury sidelining Maria Sharapova, and early losses by Lindsay Davenport and Svetlana Kuznetsova, the sisters are the only past U.S. Open women's champions left.

One Williams or the other won the tournament every year from 1999 to 2002, but neither has made so much as a final since. Less than two months after playing in the Wimbledon final, won by Venus, they could square off in the quarterfinals here.

"I've had over a week to think about it," Serena said. "So right now, I'm just hoping to win my next match."

They're also the only two American women left. Among the men, Querrey will be joined by Blake or Fish in the fourth round, while 2003 U.S. Open champion Roddick plays his third-round match Sunday against No. 31 Andreas Seppi of Italy.

That major title for Roddick, whose second-round comeback victory over Ernests Gulbis of Latvia ended after 1:30 a.m. Saturday, was the most recent by a U.S. man, a drought of 19 Grand Slam tournaments that's the longest gap for the country in the 40-year Open era.

Guys like Querrey would wake up on weekend mornings and watch Pete Sampras or Andre Agassi play in big matches at Wimbledon or the U.S. Open with regularity. That doesn't happen any more. Querrey, though, is among the latest in a long line of players expected to bring back the good times.

"He's paid his dues. I think he's really ready to move forward," Venus Williams said. "He has the experience and he's had the opportunity to play matches against all kinds of players, so I think it's now his chance to do it."

What already has been a breakthrough tournament for Querrey would really become significant with another win. He's already faced Nadal once, losing in three sets on a hard court in August 2006.

"Won the first set, I remember, so that was cool," Querrey said. "Like, the main thing from that match I remember — I was playing at 2 (p.m.), and I knew ESPN went on the air at 3. I wanted to make it to 3 to get some ESPN time, which I did. ... Hopefully I've gotten a lot better since then. He has, too."

Nadal's assessment was similar.

"I managed to catch a bit of his match," Nadal said. "He has improved a lot, and he has that powerful serve."

At 6-foot-6, Querrey is one of the few men who comes close to seeing eye-to-eye with the 6-10 Karlovic. That height and accompanying wingspan allow Querrey both to generate torque on his serves — he hit 20 aces, only four fewer than his opponent — and to handle Karlovic's own high-bouncing offerings effectively.

Karlovic's serve is "up near my shoulders," Querrey said, "where (for) other guys, it's toward their head."

Asked what about Querrey's serve is tricky, Karlovic shrugged and replied, "His height."

But there's so much more to Querrey, of course.

He has touch, as he showed by flicking a defensive lob over the tallest player in ATP history to go ahead 5-3 in the opening tiebreaker.

He has nerve, as he showed by producing 45 winners and only 13 unforced errors, and by hanging tougher than Karlovic in key moments. It was Querrey who saved break points at 4-all and 5-all in the second set. It was Karlovic who double-faulted to fall behind 6-4 in the second tiebreaker, which Querrey ended with a 127 mph ace that caught a corner.

Could Querrey beat Nadal?

"If he's going to serve like today," Karlovic said, "yeah, of course. Why not?"

Querrey couldn't have said it better himself.