NEW YORK (AP) _ One precocious teen pouted and shouted and flung her racket at the umpire's chair. The other hardly issued a peep.

In tennis, though, love means nothing and manners often count for less, and on Monday at the U.S. Open the 15-year-old who acted most churlish, Martina Hingis, came away with the tournament's biggest upset to reach the quarterfinals.

Hingis, seeded No. 16, toppled 1994 champion and No. 3 seed Arantxa Sanchez Vicario 6-1, 3-6, 6-4 in a baseline slugfest punctuated by bad calls and long arguments.

Moments later, 15-year-old Anna Kournikova threatened to continue the teen insurrection when she broke defending champion Steffi Graf's serve in the first game. It proved a short-lived revolt as Graf quelled her 6-2, 6-1, allowing Kournikova to hold serve only once.

Andre Agassi marched into the men's quarterfinals with a 4-6, 6-2, 6-3, 6-4 victory in a sloppy match against David Wheaton. Agassi, seeded sixth, goes up next against No. 3 seed Thomas Muster, who beat No. 13 Thomas Enqvist 7-6 (7-4), 6-2, 4-6, 6-1.

``It's been a good Open for me,'' said Agassi, the 1994 champion and 1995 runner-up. ``Certainly the summer has turned around since Wimbledon, winning the Olympics, winning Cincinnati and now getting this far. I'm in position to pick up the level a bit and go for the title again.''

Javier Sanchez also reached the quarters, beating Arnaud Boetsch 6-4, 7-6 (7-5), 7-6 (7-3) in a match of unseeded players.

At this juncture in their young careers, the Swiss Hingis is far more experienced in tournament play than the Russian Kournikova, who came into her first Grand Slam event as a qualifier. Yet, even in lopsided defeat, Kournikova, who has been training at Nick Bollettieri's academy in Florida since the age of 9, showed signs that in a few years she may surpass Hingis and, perhaps, everyone else.

``She has so many different shots,'' Graf said of Kournikova. ``She's gutsy. She's really eager out there. She's got a good physique, which is going to definitely work for her. Good service motion. Good backhand. Just needs to be a bit more patient. Obviously, experience is missing, but she definitely has the shots.''

Kournikova came away from her first match against Graf upset with herself though not at all discouraged.

``I learned that I could be out there and I can play those players,'' she said. ``It was really good for me. I would love to play her again and again.''

Graf next plays unseeded Judith Wiesner, who beat Rita Grande 6-0, 6-3.

This is Hingis' eighth major event and as far as she's gone _ matching a quarterfinal finish at the Australian Open this year. She's still a long way from the talent of the original Martina, for whom she was named. But Hingis is steadier, stronger and more confident than ever, and when she contains her temper she is a formidable baseliner.

``It's a very talented, intelligent game she's playing out there,'' Graf said of Hingis, who could meet her in the semifinals. Hingis must first get past Jana Novotna, a 6-2, 6-0 winner over Karina Habsudova on Monday.

``She knows exactly what to do,'' Graf continued about Hingis. ``Maybe she doesn't have that all-court game that some of us have, but she's using the court really well. She makes you play a lot of shots that maybe you shouldn't try for. But because she plays them really close to the lines, shorter ones, she knows really well what to do on the court. She won't try for too-crazy shots.''

Against Sanchez Vicario, a finalist at the French, Wimbledon and the Olympics, Hingis pressed the attack with deep, punishing groundstrokes and occasional forays to the net on short balls. She is, in that way, very similar to Sanchez Vicario, and in this match the Spaniard took fewer risks and made more mistakes, slapping 52 unforced errors to Hingis' 33.

``I didn't feel any pressure because I played a 15-year-old,'' Sanchez Vicario said. ``She's just a very talented player. She has very good hands. And the first set I was not on the court at all.''

Hingis' weakness is her concentration, which drifts sometimes just when she most needs it. She bounces her racket off the court in anger at times, drops it in disgust other times, and occasionally flings it to the side.

Upset at an overrule that went against her, she tossed her racket 20 feet to the base of the umpire's chair after Sanchez Vicario took a 4-3 lead in the second set. Hingis had a chance to go to break-point in that game, but umpire Jane Harvey called her groundstroke long. Televised replays clearly showed the umpire was wrong, but Hingis let the mistake rattle her.

After her display of petulance, Hingis played a half-hearted service game, was broken at love to 5-3, and wound up dropping the set.

Still in a funk, Hingis lost the first two games of the third set and seemed at that point to be on her way out of the tournament. But with the benefit of youth on a very hot day, Hingis worked Sanchez Vicario from side to side in the next game, tiring her out so much she never really recovered.

Hingis swept to a 3-2 lead by winning one long rally after another, and two games later the bad calls evened out when Harvey overruled a game-point groundstroke by Sanchez Vicario in Hingis' favor. Again, replays indicated Harvey was wrong.

Fatigue affected Sanchez Vicario more than that call, and she slapped three straight unforced errors to fall behind 5-3. Hingis was getting tired, too, cramping a bit in her legs as she served for the match. She dropped her racket in annoyance again when she netted an easy forehand to fall behind love-40, then struck a cream puff second serve that Sanchez Vicario pounced on for a winner.

But that was about all Sanchez Vicario had left. At 15-40 on her serve, the end was delayed a moment when a point had to be interrupted and replayed after a plastic bottle blew toward the court in the gusting wind. When they resumed, Sanchez Vicario sailed a backhand slice long to leave in the fourth round for the second straight year.