Roger Federer opened his ATP Rogers Cup campaign with a straight-sets second-round victory over Juan Ignacio Chela in Toronto on Tuesday night but Andy Roddick withdrew from the event due to illness.
The 29-year-old Swiss, who is seeded third and received a first-round bye, saw off his Argentinian opponent 7-6 (9/7) 6-3 in his first match since his quarter-final exit at Wimbledon.
Roddick, meanwhile, pulled out of the competition citing illness and was replaced in the draw by lucky loser Paul-Henri Mathieu. The Frenchman will take on Lu Yen-Hsun in the second round on Wednesday after the Taiwanese player saw off Canada's Pierre-Ludovic Duclos 6-4 6-4.
Argentina's David Nalbandian secured his place in round two by ousting 10th-seeded Spaniard David Ferrer 7-5 3-6 6-3.
Eleventh seed Marin Cilic was also eliminated, the Croatian going down 6-4 7-6 (7/2) to Viktor Troicki of Serbia.
Ninth seed Fernando Verdasco of Spain fared better, however, prevailing 6-0 5-7 6-2 in a topsy-turvy encounter with Argentina's Eduardo Schwank.
Sixteenth seed Sam Querrey also required three sets to get past fellow American Michael Russell, 3-6 6-1 6-4, but 15th seed Gael Monfils of France had no such troubles in despatching India's Somdev Devvarman 7-6 (7/3) 6-3.
Belgium's Xavier Malisse set up a second-round meeting with defending champion Andy Murray by beating Germany's Michael Berrer 7-6 (7/2) 6-2.
Serbian Janko Tipsarevic retired with an ankle injury in the early stages of his match - 2-2 in the first set - against Germany's Philipp Kohlschreiber.
Elsewhere in the first round there were wins for French duo Jeremy Chardy and Michael Llodra, Switzerland's Stanislas Wawrinka, Italy's Fabio Fognini and Ukraine's Alexandr Dolgopolov.
In second-round action, fifth seed Robin Soderling and seventh seed Tomas Berdych advanced with victories over Ernests Gulbis and Sergiy Stakhovsky respectively.
Soderling came from behind to overcome Latvian Gulbis 4-6 6-4 6-4 while Berdych cruised past Ukraine's Stakhovsky 6-2 6-4.