Venus Williams was upset 6-4, 7-6 (7-5) by 25th-seeded Lisa Raymond in the third round of the Australian Open.
The third-seeded Williams saved one match point with a backhand crosscourt winner at 4-6 in the tiebreaker, but put a forehand return wide on the next point to end the match.
"I'm pretty much in shock," Williams said. "I definitely had some high hopes to win here."
It was the first time Williams hasn't reached the quarterfinals in six trips to the Australian Open. She lost last year's final to her sister, Serena.
It was the biggest upset at the Australian Open since Jennifer Capriati, then defending champion, lost in the first round last year to Marlene Weingartner.
Williams, a four-time Grand Slam champion, was sidelined with an abdominal injury for six months and was returning to tournament play for the first time since losing the Wimbledon final to Serena last July. She played two exhibition matches in Hong Kong before coming to Melbourne and having straight-sets wins in the first two rounds.
Asked if she was not 100 percent, Williams said: "Everybody has their issues. I don't want to get into it."
For Raymond, a former Gator, it was her first win over Venus Williams, who gave a brief wave to the crowd and quickly walked off the court.
Raymond, a former top-ranked doubles player, said it was the match of her life.
"I couldn't ask anything more of myself today," she said. "I think I played an incredible match.
"It's pretty difficult to play someone who is serving like that today. I came into this match knowing I had to step up to the plate and go for it _ I stepped up to the plate and I believed. It's the only way I was going to win."
Williams had 44 unforced errors. Four of her seven double-faults were in the first two games. She got her serve back on track and had 14 aces and 40 winners, but didn't have a match for Raymond's pinpoint forehands.
Raymond, 30, had 23 winners, including 14 on her forehand side. She broke Williams twice to take a 3-0 lead and forced an error to win the first set on her fifth set point.
The pair traded breaks to open third before Raymond held to lead 3-1.
Williams finally got her serve working, firing three aces in the next game and then breaking Raymond to level at 3-3.
She served another two aces, including one at 120 mph to take a 4-3 lead, but Raymond won her next service game at love.
Williams opened the tiebreaker with an ace before Raymond won the next four points. Williams leveled it at 4-4 with an ace. She served a fault, a let and then Raymond pounced on the second serve with a backhand that forced an error to establish a 5-4 lead.
"In the tiebreak I probably went for too much, too soon," Williams said. "I guess I'll have a lot of time to think about it now."
Raymond had never gone beyond the third round at Melbourne Park. Her best performance in a Grand Slam tournament was the quarterfinals at Wimbledon in 2000.
Raymond will next play French teenager Tatiana Golovin, who upset 23rd-seeded Lina Krasnoroutskaya 6-2, 7-6 (4).
Russian-born Golovin, who will turn 16 Sunday, had won only one match on the WTA Tour before her first three wins here at Melbourne Park.
Williams' shocking loss overshadowed wins by other high-profile players, including No. 2-ranked Kim Clijsters' 6-2, 6-1 win over Russia's Dinara Safina.
Wimbledon champion Roger Federer beat teenage wild-card entry Todd Reid 6-3, 6-0, 6-1.
Federer, seeded second, dropped serve in the fifth game of the first set, then reeled off 14 consecutive games to win the first two sets and take a 4-0 lead in the third.
The 22-year-old Swiss star won eight of the last nine points to finish off the match in 74 minutes.
Reid, 19, struggled through the second round in a grueling five-set win over Armenia's Sargis Sargsian.
Federer didn't give him any reprieve, firing 31 winners and earning his third straight-sets win in three rounds to match his best performance in the season-opening Grand Slam event.
"I played well, had my difficulties in the start, but it went better in the end," Federer said. "I'm just happy to be again through to the fourth round."
Top-ranked Andy Roddick was just as ruthless against Taylor Dent, who did little but chase balls nonstop during a humbling 71 minutes of tennis. Roddick won 6-2, 6-0, 6-2.
Dent called the loss "without question, the worst tennis experience of my entire life."
"I wasn't tanking out there. I was busting my butt. It was embarrassing, absolutely embarrassing," he said.
Defending champion Andre Agassi was equally overpowering. The eight-time Grand Slam champ routed Sweden's Thomas Enqvist 6-0, 6-3, 6-3 to extend his winning streak in the Australian Open to 24 matches _ spanning titles in 2000, '01 and '03.
"I feel like I did everything I was looking to do," Agassi said.
Top-ranked Justine Henin-Hardenne survived her first serious challenge, losing her serve twice in the second set before taking four consecutive games to beat Svetlana Kuznetsova 6-2, 7-5.
Federer will face either Lleyton Hewitt, a former U.S. Open and Wimbledon champion, or 17-year-old Spaniard Rafael Nadal.
MEN: Andy Roddick (1) def. Taylor Dent (27) 6-2, 6-0, 6-2; Andre Agassi (4) def. Thomas Enqvist 6-0, 6-3, 6-3; Sebastien Grosjean (9) def. Dominik Hrbaty 2-6, 6-4, 6-1, 6-3; Paradorn Srichaphan (13) def. Gustavo Kuerten (19) 6-3, 7-5, 6-4; Sjeng Schalken (16) def. Jurgen Melzer 7-6 (7-1), 6-4, 6-1; Robby Ginepri (32) def. Nicolas Escude 6-2, 6-3, 6-4
WOMEN: Justine Henin-Hardenne (1) def. Svetlana Kuznetsova (30) 6-2, 7-5; Lisa Raymond (25) def. Venus Williams (3) 6-4, 7-6 (7-5); Amelie Mauresmo (4) def. Anabel Medina Garrigues 6-1, 6-2; Lindsay Davenport (5) def. Laura Granville 6-4, 6-0; Vera Zvonareva (11) def. Nicole Pratt 7-5, 2-6, 6-1.
TODAY ON TV: Early rounds, noon ESPN2 (taped); 10 p.m. ESPN2.