Andre Agassi began his quest for a second successive title with an emphatic 6-4, 6-2 win over Spain's Alberto Martin in the first round of the Montreal Masters on Monday.
Tim Henman, however, became the highest seeded casualty on the first day of the event when he was tamed 3-6, 7-5, 6-1 by Slovakia's Dominik Hrbaty.
Agassi, who marked his comeback from a two-month injury layoff with victory in Los Angeles eight days ago, fired down eight aces to easily overcome Martin.
"I was pleased with the way I improved over the course of the match," said the 35-year-old American fourth seed.
"I was a little nervous in the beginning and the ball was really flying ... and I was losing control of a few shots. But by the end, I started finding my swing again and felt pretty good."
Tenth seed Henman appeared to be coasting towards a place in the second round before Hrbaty suddenly came to life to bamboozle the Briton.
Despite the defeat, Henman remained upbeat about his chances at the U.S. Open, which begins on August 29.
"It's frustrating but not alarming," said Henman, who arrived in Canada after a family holiday in Tuscany.
"I played well in the first set but when my level of play dropped, his rose.
"I'm trying to improve certain aspects of my game and I'm going to continue to put into play in matches what I work on in training."
Former French Open champion Gaston Gaudio led a trio of Argentines into the second round.
Sixth seed Gaudio defeated Italy's Davide Sanguinetti 1-6, 7-6, 7-5, eighth seed Mariano Puerta edged out Luis Horna 7-6, 7-6 and ninth seed David Nalbandian beat countryman Juan Ignacio Chela 6-4, 6-1.
