The tear in the cartilage in Pete Sampras's lower back is only two millimeters long, but it was big enough to throw the U.S. Open into turmoil today when the top seed was forced to drop out.

Coming on the same day that two-time defending champion Patrick Rafter retired from his first-round match with a shoulder injury, Sampras's departure ends his bid at a record 13th Grand Slam title, wrecks a possible showdown with fellow American Andre Agassi and wipes out any chance he had to hold on to the world's No. 1 ranking.

"The last three days have been hell -- the worrying about not playing and the stress," he said. "Then we finally got some tests done on it and that was really that last straw that said, `That's it, it's over.' "

Sampras had been on a course to meet Agassi in what would have been a blockbuster final, but his back injury combined with Cedric Pioline's victory over the fourth-seeded Rafter threw open the upper half of the draw.

"It's tough for the tournament, especially with Pete playing such great tennis," Rafter said. "For me, it was very tough to pull myself off that court."

Rafter won the first two sets, 6-4, 6-4, but a shoulder injury that has been bothering him for the last three months began to flare up in the third set, which Pioline took, 6-3. Rafter received treatment from a tournament trainer on changeovers and seemed to improve in the fourth set, even coming within two points of winning the match.

But he was never able to break Pioline again. Pioline took the fourth set, 7-5, and when he broke Rafter in the first game of the fifth set, Rafter retired.

This is the first time in the 119-year history of the U.S. nationals that the men's defending champion has lost in the first round.

"I was very surprised because I know Patrick is a very good fighter -- he never gives up, so if he stopped the match, it's because he had a very serious problem," Pioline said. "His shoulder was sore and his serves were not as powerful [in the third set], so I started to be more aggressive and more confident."

Sixth-seeded Tim Henman and No. 13 seed Alex Corretja also were ousted, as was U.S. phenom Alexandra Stevenson, who lost her opening-round match to No. 11 seed Nathalie Tauziat, 6-2, 6-2. Lindsay Davenport (2), Monica Seles (4) and Mary Pierce (5) advanced, but their matches were overshadowed by the news about Sampras.

"I was sad -- I heard it when I was waiting to play," Davenport said. "It's a real blow to the tournament, to the fans and even to the players."

Sampras suffered the injury on Sunday, feeling "something give" at a practice session with No. 5 seed Gustavo Kuerten. Sampras has had health problems here before -- last year he retired from his semifinal against Rafter with an injured thigh, and in 1996, he vomited on the court during a five-set victory over Corretja.

But even on Sunday, Sampras felt his injury might be more serious, despite a diagnosis of back spasms by tournament doctors.

"It was on a return of serve," Sampras said. "I felt like my back was stabbed by a knife, and I immediately stopped. It scared me; there's no question it scared me. I just walked off the court."

Sampras canceled his practice session on Monday and by that evening was having trouble moving around his hotel room. Brian Hainline, the tournament's medical director, ran him through a series of tests, one of which revealed a slight tear in the cartilage between the spine's vertebrae. The injury will not require surgery, but Sampras will not be able to play competitively for one to two months.

"We were fortunate in being able to diagnose the herniated disk very early, before it became anything serious," Hainline said. "We would expect him to recover fully from this."

While Sampras was pleased to hear the injury was not career-ending, he was devastated that its timing will prevent him from competing here. He had been hoping to capture his 13th career Grand Slam, something no man has ever accomplished. He tied the record held by Roy Emerson by winning his sixth Wimbledon title in July, and he had not lost a match since.

The injury also will force him out of the No. 1 ranking, with either No. 2 Agassi or No. 3 Yevgeny Kafelnikov poised to take the top spot. But while Agassi stands to benefit from Sampras's troubles, he is not pleased to see his rival go. He even called Sampras at his hotel this morning to offer regrets, which Sampras called "a very classy move on his part."

Sampras will go home to California for rest and rehabilitation. He said it will be too painful to watch the tournament on television, although he will spend some time trying to glean some meaning from what has happened to him.

"I believe everything happens for a reason, and I've been trying to figure out what it is," Sampras said. "But right now, I'm obviously very overwhelmed. I really wanted the chance to play here."

U.S. OPEN

When: Today-Sept. 12.

Where: USTA National Tennis Center, Flushing Meadows, N.Y.

TV: USA, 11 a.m., 7:30 p.m.

Defending champions: Patrick Rafter, Lindsay Davenport.

Top seeds: Pete Sampras, Martina Hingis.

Yesterday's results: Men -- Guillermo Canas, Argentina, def. Tim Henman (6), Britain, 7-6 (7-1), 6-4, 6-3; Gustavo Kuerten (5), Brazil, def. Bohdan Ulihrach, Czech Republic, 6-4, 6-4, 6-3; Todd Martin (7), United States, def. Stephane Huet, France, 6-4, 6-3, 6-7 (7-2), 6-7 (7-5), 7-6 (7-3); Nicolas Kiefer (15), Germany, def. Jason Stoltenberg, Australia, 6-4, 6-3, 1-6, 4-6, 6-3. Women -- Lindsay Davenport (2), United States, def. Corina Morariu, United States, 6-0, 6-3; Mary Pierce (5), France, def. Sandra Nacuk, Yugoslavia, 6-3, 7-5; Julie Halard-Decugis (9), France, def. Cara Black, Zimbabwe, 6-3, 5-7, 6-3; Jennifer Capriati, United States, def. Iva Majoli, Croatia, 6-1, 7-5; Samantha Reeves, United States, beat Lilia Osterloh, United States, 7-5, 6-3.

Today's featured matches: Men -- Greg Rusedski (9), Britain, vs. Juan Carlos Ferrero, Spain; Thomas Enqvist, Sweden, vs. Tommy Haas (14), Germany; Max Mirnyi, Belarus, vs. Yevgeny Kafelnikov, Russia. Women -- Martina Hingis (1), Switzerland, vs. Sarah Pitkowski, France; Venus Williams (3), United States, vs. Anne-Gaelle Sidot, France; Tina Krizan, Slovenia, vs. Jana Novotna (8), Czech Republic; Nicole Pratt, Australia, vs. Arantxa Sanchez-Vicario (10), Spain; Amelie Mauresmo (15), France, vs. Mariaan de Swardt, South Africa.