The 36-year-old Agassi fired 17 aces and hammered 28 forehand winners to defeat Romania's Pavel 6-7 (4/7), 7-6 (10/8), 7-6 (8/6) 6-2 in three hours, 31 minutes in front of a crowd of 23,736 at the Arthur Ashe Stadium.

For while it looked like Agassi might be calling it a career as he had already announced this would be his last tournament before retirement.

"I know this was going to be the toughest one (match) as far as the emotional side of things go." Agassi said.

"You want it to be everything you hoped it is going to be but you are not even sure what that is."

Tied 1-1 in sets and down 4-0 in the third set, the unseeded Agassi dug deep eventually winning in a tie-break.

"It was looking pretty bleak in the third set," Agassi said. "But I got through it."

Agassi now moves onto the second round where he will play an even tougher opponent in eighth seeded Marcos Baghdatis of Cyprus.

World No. 3 Ivan Ljubicic suffered the first major upset on a rain-hampered opening day that saw the rest of the top seeds breeze into the second round.

The tournament got off to a late start as two days of heavy rains pushed back the start times of Monday's early matches. The rain stopped in the afternoon but more showers are in the forecast for Tuesday.

Sixth seeded Tommy Robredo and ninth seed Andy Roddick easily advanced to the second round Monday with straight set victories.

Robredo rolled past Brazil's Flavio Saretta 6-2, 6-1, 6-2 while Roddick overpowered Arnaud Clement of France 6-2, 6-1, 6-3.

Baghdatis reached the second round with a comfortable enough 7-6, (7/1), 7-6, (9/7), 6-3 win over Germany's Alexander Waske.

On the women's side of the draw, second seeded Justine Henin-Hardenne defeated Maria Elena Camerin, of Italy, 6-2, 6-1. Russian seeds Elena Dementieva and Svetlana Kuznetsova, who contested the 2004 final, also made it to the next round.

Ljubicic was sent crashing out of the tournament, dropping a 6-3, 6-3, 6-3 match to unseeded Spaniard Feliciano Lopez.

This was the fifth career and fourth meeting this year between Ljubicic and Lopez, who has now won the last two. Lopez beat Ljubicic 7-6, 6-3 last month in Gstaad.

"When I saw the draw I knew it would be tough but I didn't expect to lose this easy," the 27-year-old Croatian said. "I have had problems with him in the past. He was returning my serves really easily."

Ljubicic struggled with his serve during the one hour, 41 minute match.

He blasted 13 aces but made more unforced errors (27-23) than Lopez, who beat Ljubicic for the first time in a Grand Slam after losing twice in 2006 to him.

"It just wasn't my day," Ljubicic said.

Roddick, who won his first tournament of the season last week in Cincinnati, unleashed a 238kph (148mph) serve and fired 10 aces as he routed Serra.

Roddick, the 2003 US Open champ, advanced to the second round where he will face the winner of Spain's Alberto Martin and Kristian Pless of Denmark.

"I feel like I've been hitting the ball pretty well," said Roddick, who suffered a shocking first round exit last year to Gilles Muller. "It is a lot better than last year."

Five-time Grand Slam winner Henin-Hardenne's march towards reclaiming her US Open title began in earnest as she cruised past Camerin.

The reigning French Open champion made quick work of her opponent, taking just 74 minutes to dispatch the Italian at the Arthur Ashe Stadium.

Henin-Hardenne, the 2003 US Open champion, is an impressive 44-6 heading into the final Grand Slam of the season.

"I feel good. This is the first year since 2003 that I'm playing the four Grand Slams in the season," Henin-Hardenne said.

American Lindsay Davenport, who won the 1998 US Open, rolled to an easy 6-1, 6-4 first round victory over Klara Zakopalova.

Davenport was relieved she didn't have to stay on the court longer, winning in just 52 minutes.

"The most important thing was to get off to a good start," Davenport said.