The victory for Muster, a former world No 1 himself, now ranked 20th, means the Australian Open champion, Petr Korda, can take top place with a win at Indian Wells. Korda, the second seed, beat the German teenager Tommy Haas 7-6, 6-2 to reach the quarter- finals on Thursday.
Sampras was not the only big name to fall in the third round. Andre Agassi, who has worked his way up to 40th in the world from 141st last November, beat the third-seeded Australian Pat Rafter, the reigning US Open champion, 6-3 3-6 6-2. Agassi will face the lowest-ranked player left in the draw, the 126th-ranked wild card, Jan-Michael Gambill.
But Sampras' defeat, by a player who had won just one of his three matches all year, eclipsed that upset. "I was awful," admitted Sampras. "I really struggled throughout the whole match. I had some chances in the first, had a set point. Had some love-30 points."
Sampras, the Champions' Cup winner in 1994 and 1995 but a beaten quarter- finalist in 1996 and in the second round last year, said he still had not mastered the desert conditions. "I don't know what it is about the conditions here," he said. "The ball seems to fly on me. I can't play the way I want to play, can't swing the way I want to swing, and have control of the ball."
Muster, who disputed the world No 1 place with Sampras in 1996, and grabbed the top spot for six weeks that year, said he could tell Sampras was struggling. He said: "You always play like your opponent allows you to play. But he wasn't serving as accurate as he used to. He made a lot of unforced errors. For whatever reason I don't know."
In the quarter-finals, Muster will face the Ukrainian Andrei Medvedev, who beat Nicolas Lapentti of Ecuador 6-1, 6-4. Korda will play the seventh- seeded Chilean Marcelo Rios in a repeat of the Australian Open final. Rios defeated German Nicolas Kiefer 6-4, 6-3.
The sixth-seeded Briton, Greg Rusedski, advanced with a 6-3, 7-5 victory over Spain's Carlos Moya, setting up a clash with unseeded Swede Thomas Enqvist, a 6-2, 7-6 winner over Bohdan Ulihrach.
"I was very pleased with the way I played," said Rusedski, who had a first-round bye before struggling to a 4-6, 6-3, 6-4 victory over Vince Spadea. I serv ed a lot better, played a more solid overall game and fixed up a few areas I wasn't pleased with."
Rusedski, who has beaten the 25th-ranked Enqvist in three of their four previous encounters, served 12 aces and Moya never had a chance to break his opponent's serve.
A strained left hamstring ruined Steffi Graf's latest comeback from injury when she retired in the third set of her Evert Cup semi-final match with Lindsay Davenport. Graf, who has had a long list of ailments during her career, was trailing 6-4, 4-6, 4-2 to Davenport, the defending champion. Graf, 28, was playing in only her second tournament after taking nine months off after knee surgery.
"It's like, what else?" said Graf, who has numbered a bad back, bone spurs in her feet and the broken cartilage and torn tendon in her left knee that forced the surgery last June among her injuries.
A hamstring strain was a new experience. "This one I haven't had before," said Graf, who felt a pain after a sudden movement in the fifth game of the third set. She winced and clutched her thigh, but finished the game. After having her thigh strapped during the change-over, she tried to play, earning a break point against Davenport in the sixth game even though she was hobbled by the pain.
"The minute it happened I knew," said Graf. "I always try. I almost made the game. I almost had a chance. But just standing there I was scared to make it worse."