Having arrived at Flushing Meadow following a six-week break to nurse an injured left ankle, Sampras appeared invincible until the unfancied Roger Smith put him under pressure in the third round, taking a set off the world No 1.
Yzaga then produced the upset of the championships. A 3-6, 6-3, 4-6, 7-6, 7-5 win made him only the seventh player to defeat Sampras in 66 matches this year, and only the second after Jim Courier (in the quarter-finals of the French Open) to deny him in a Grand Slam tournament.
The extent of Sampras's distress became apparent after the opening game of the fourth set, when he requested a time-out for treatment. This was refused because the problem was regarded as conditioning, not injury, and the Californian had to wait to receive treatment during change-overs.
Hanging his head, and barely able to move on certain points, let alone serve anywhere close to capacity, Sampras dragged himself into a fourth-set tie-break, which he lost, 7-4, after taking the opening two points.
After that, the outcome depended upon Sampras's will- power and the strength of Yzaga's nerve. The 26-year-old from Lima, ranked No 23, broke for 4-2, only to lose his serve with the match there for the taking at 5-4.
It appeared that Sampras would force the contest into a deciding tie-break, but some spectacular cross-court winners were interspersed with weary errors, and Yzaga scored the winning point with an emphatic backhand pass after three hours and 37 minutes.
'He hadn't played in six weeks. You need to play matches. You can't get that on an exercise bike,' Tim Gullikson, Sampras's coach, said. 'After the third set I thought he was going to pack it in but I've never seen him do that. He looked as if he was going to pass out. I can't fault him for his effort.'
Yzaga now plays Karel Novacek, a 29-year-old ranked No 56 for a place in the semi-finals. With Sampras gone, two other Americans, the ninth-seeded Todd Martin and the unseeded Andre Agassi, will be fancying their chances. For the first time the three top-seeded men have been eliminated before the quarter- finals.
The first fresh face through the the quarter-finals was Sweden's Jonas Bjorkman, the conqueror of Stefan Edberg, who defeated Germany's Joern Renzenbrink, 3-6,
6-3, 6-2, 6-7, 6-3.
The women's singles continues to chug along. Oops] A 49mph serve fails to reach the net. Oops again] One timed at 46mph also lands on the server's side. This is Gabriela Sabatini en route to the semi-finals.
The eighth seed speaks of being a better player 'technically' than she was in 1990, when she defeated Steffi Graf here to win her only Grand Slam title, and yet most of her serves continue to have the impact of a duster.
As Gigi Fernandez, her opponent yesterday, said: 'My mother can serve harder than that - the worst part is I was missing the returns.' Which is one of the reasons why Sabatini was able to escape from 0-3 in the second set and survive having her serve broken when serving for the match at 5-4 to win, 6-2, 7-5.
Perhaps Fernandez's mother would have been able to staunch a run of 13 points against her in the opening set and the loss of 14 in a row from 3-0, 40-0 in the second set.
Sabatini now plays Arantxa Sanchez Vicario, the second seed, who has won the French Open since beating the Argentinian, 6-1, 6-2, in the semi-finals of the Australian Open in January.
The Spaniard, who has yet to drop a set, had a comfortable quarter-final against Kimiko Date, of Japan, capitalising on the fifth seed's 39 unforced errors to win,
Billie Jean King, the former Wimbledon champion, is ready to launch a new women's tour next year to be organised in conjunction with Mark McCormack's International Management Group.
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