In his first match since lifting a record 15th Grand Slam singles title at Wimbledon, much has changed in a few weeks for Federer, who arrived in Montreal with his wife Mirka, new born twin daughters Charlene Riva and Myla Rose and a nanny in tow.
Third seed Andy Murray and number four Serb Novak Djokovic, also back in action for the first time since Wimbledon, advanced smoothly.
Briton Murray, who can snatch the number two world ranking from Nadal with a decent run in Montreal, beat Frenchman Jeremy Chardy 6-4, 6-2.
Serbia's Djokovic battled past Canadian wildcard Peter Polansky 6-4, 7-6.
Eighth-seeded Russian Nikolai Davydenko also advanced with 7-6 7-6 win over Frenchman Paul-Henri Mathieu.
World number 10 Fernando Gonzalez of Chile survived a scare, battling back to beat 679th-ranked Canadian Milos Raonic teenager 4-6, 7-6, 6-4 in a first-round meeting.
Mahesh Bhupathi and Mark Knowles advanced to the pre-quarterfinals event after getting a bye in the first round.
The Indo-Bahamian pair, seeded third, will now take on Jeff Coetzee of South Africa and Jurgen Melzer of Austria for a place in last-eight.
Safina survives
World number one Dinara Safina scrambled past Italian Roberta Vinci 2-6, 7-5, 6-4 to reach the third round of the Cincinnati Open on Tuesday.
French Open champion Svetlana Kuznetsova also survived a scare and two rain breaks to beat Czech Lucie Safarova 6-3, 6-7, 7-6.
Third seed Venus Williams drillec eight aces as she crushed Olga Govortsova of Belarus 6-2, 6-2. Italian 14th seed Flavia Pennetta scored a crushing 6-2, 6-3 first-round victory over Japan's Ayumi Morita. In other first-round action, Daniela Hantuchova recovered from a slow start to beat Italy's Sara Errani 3-6, 6-3, 6-1 in a match lasting more than two and a half hours.