Jennifer Capriati was sensationally knocked out of the Australian Open by Germany's Marlene Weingartner on the opening day of the year's first grand slam event .

The third-seeded American's hopes of winning a hat-trick of titles at Melbourne Park came to an abrupt halt on Monday when she blew a commanding lead to go down 2-6, 7-6, 6-4 in the first round to a player ranked 87 places below her.

Capriati looked to have the match under control when she raced through the first set then opened up a 4-2 lead in the second. But her lack of match practice suddenly caught up with her and her game fell apart at the seams.

She failed to hit her serves with any consistency and started spraying her groundstrokes all over the court as Weingartner, sensing her opportunity, pounced.

The German captured the second set tiebreaker 8-6 and snatched victory when she broke Capriati's serve on her first match point to pull off the biggest upset of her career.

Capriati, looking heavier and slower than when she saved four match points to beat Martina Hingis in last year's final in one the great comebacks of tennis history, said she was not fit enough this time.

"I felt like I didn't give myself the proper preparation," Capriati said.

"If I wasn't the defending champion, I probably wouldn't have shown up."

Capriati's stunning defeat has increased the chances of another all-Williams final after Venus swept past Svetlana Kuznetsova 6-4, 6-2.

With little sister Serena due to begin her campaign for a non-calendar year grand slam on Tuesday, Venus showed she is still looming as the biggest threat with an effortless win.

BULLDOZED

Andre Agassi also showed why he is being tipped to win a fourth title in the men's competition after he bulldozed fellow American Brian Vahaly 7-5, 6-3, 6-3.

The three-times champion spent just over two hours booking his next round spot. He had been unable to defend his title here last year because of a wrist injury and looked thrilled to be back on court, blasting 36 clean winners.

"To be back here... there is a lot of nerves," Agassi smiled courtside afterwards. "It's been a while."

Champion in 2000 and 2001, Agassi has not lost at Melbourne Park since a fourth round loss to Vince Spadea in 1999.

"At this stage I am very physically prepared to do whatever is needed of me here," he said. "I feel good at slams. I feel good in my matches."

Once Venus had shaken off the ring-rustiness caused by two months away from competitive action she also ran away with her match, demolishing the gifted Kuznetsova.

"I've been practising, going out, living life," she giggled when asked of her preparations. "I was just a little rusty out there but at times I was my old self.

"I didn't expect to be 100 percent in this match but next match I'll be 150."

CONSIDERED RETIRING

Russian Yevgeny Kafelnikov safely booked his place in the second round with a 7-6, 6-3, 4-6, 6-3 win over Jeff Morrison.

Australian Open champion in 1999 and runner-up in 2000, Kafelnikov, 28, had considered retiring at the end of last year after helping Russia win the Davis Cup for the first time but decided to keep playing.

Just four of the 32 seeds in action fell by the wayside on Monday, with former Wimbledon champion Conchita Martinez joining Capriati on the casualty list.

Martinez, the women's 31st seed, lost to Australia's Samantha Stosur 6-7, 6-3, 6-4.

South African Wayne Ferreira sent Spanish 26th seed Tommy Robredo packing with a 5-7, 6-0, 6-3, 6-4 win and Romanian 21st seed Andrei Pavel retired after aggravting a back injury in the first set of his match with Renzo Furlan.

PERFECT MATCH

Anna Kournikova was won her first grand slam match in two years, sweeping past Henrieta Nagyova 6-1, 6-2.

"I played nearly a perfect match today," she smiled. "It didn't cross my mind that I lost four (first round) grand slam matches in a row. It's not good if you think about that too much."

The victory sets up a second round clash with fifth seed Justine Henin-Hardenne -- her conqueror in the first round here last year.

The newly-married Belgian breezed into the second round with a 6-3, 6-4 win over Switzerland's Myriam Casanova.

Asia's two in-form player both kicked off their capaigns with victories. Thailand's Indian Open champion Paradorn Srichaphan beat Jurgen Melzer of Austria 7-5, 6-4, 1-6, 6-0 and last week's Sydney winner, South Korean Lee Hyung-taik, defeated Spain's David Ferrer 5-7, 6-2, 6-2, 6-3 to set up a second round meeting with Agassi.

Women's ninth seed and former champion Lindsay Davenport beat French wildcard Camille Pin 6-2, 6-1, seventh-seeded Slovak Daniela Hantuchova beat Colombia's Fabiola Zuluaga 7-5, 6-7, 6-4.