MONTREAL (Ticker) -- Roger Federer had his hands full in his return to the court. After a difficult opening set, Rafael Nadal had an easy go of it.
Defending champion Federer opened his road to the US Open with a 7-6 (7-2), 7-6 (7-3) victory over Croatian Ivo Karlovic in the second round of the Rogers Cup on Wednesday.
Meanwhile, the second-seeded Nadal withstood an early challenge to post a 7-6 (7-4), 6-0 triumph over another former champion in Marat Safin in the hardcourt tournament.
For the top-seeded Federer, this $2.45 million event was his first action since capturing a record fifth straight Wimbledon title in five sets over Nadal in July.
"When you have three, four weeks ahead of you without any tournament, you kind of just relax, you enjoy the moment," Federer said. "After you start practicing again, you kind of forget what happened, you look forward and you arrive here, all of a sudden, everybody talks about Wimbledon again. For me, it's already kind of in the past."
The Swiss star took the same break last year before winning this event en route to a third consecutive US Open title.
Federer had a tough time dealing with the hard-serving Karlovic, who finished with 16 aces against just one double fault.
"I couldn't read his serve at all," Federer admitted.
However, Federer - who turned 26 on Wednesday and was presented with a cake following the win - came up with the key points in the tiebreakers en route to improving to 3-0 against the 6-10 Karlovic.
"I think I played very well, actually," Federer said. "I didn't give many chances on my own service game. From the baseline, I hardly missed a shot. I played aggressive."
In Wednesday night's feature match, Nadal overcame 11 aces by Safin to emerge victorious in the first meeting between the two. The 2000 champion of the event, Safin recorded eight of his aces in the first set of a match that lasted just over 1 hour, 40 minutes while winning 84 percent of his first serves.
Nadal managed to win the tiebreak, however, and rolled through the second set, capturing 85 percent of his initial serves while winning 93 percent of second-serve returns.
"Today is difficult to play very good because Marat is touching the ball with unbelievable power every time," Nadal said. "But I am there, I fight."
Nadal already has returned to action since finishing as the runner-up at the All England Club for a second straight year, collecting his sixth title of the season on the clay at Stuttgart, Germany in July.
A winner here in 2005 when Federer was sidelined with an injury, the Spanish Nadal is playing on a hard court for the first time since a quarterfinal finish at the Masters Series event in Miami in March.
Nadal will take on Frenchman Paul-Henri Mathieu in the third round. Mathieu, who is winless in five previous meetings with Nadal, posted a 6-3, 6-2 victory over Mario Ancic of Croatia.
"I know I have to improve something for tomorrow," Nadal said. "I have to improve the serve. I have to improve play, little bit more aggressive with the forehand. ... I'm going to work on that tomorrow morning."
The top eight seeds all received first-round byes.
Despite being caught up in a whirlwind due to gambling allegations about his retirement from a second-round match against Martin Vassallo Arguello in Sopot, Poland last week, Nikolay Davydenko proved unrattled on the court.
The fourth seed moved into the third round with a 6-3, 7-6 (7-5) victory over Finland's Jarkko Nieminen.
The Russian has been allowed to continue to play in the midst of the ATP's investigation into the matter.
Davydenko next will play countryman Mikhail Youzhny, a 6-4, 6-4 winner over Stanislas Wawrinka of Switzerland.
Top American Andy Roddick moved on with a 6-1, 7-6 (7-5) victory over Arnaud Clement. The fifth seed leveled his career mark against the Frenchman at 2-2, remaining perfect in meetings on hard courts.
The world's No. 3, Roddick next will face Cyprus' Marcos Baghdatis, a 6-2, 3-6, 6-1 winner over Dutch qualifier Robin Haase.
The other main player in the American contingency, ninth seed James Blake was forced to withdraw with an abdominal strain prior to the start of his match against former US Open champion Lleyton Hewitt of Australia.
The injury continued a disappointing showing here for Blake, who has failed to make it past the second round in four appearances.
Hewitt, the 2001 US Open champion, could meet Federer in the fourth round.
Two top seeds that did not make it past the second round were seventh seed Tommy Robredo of Spain and No. 8 Richard Gasquet of France.
Robredo had a difficult transition from clay, absorbing a 6-2, 6-4 loss to Dominik Hrbaty of Slovakia. It was the first meeting between both players.
Prior to his first hardcourt tournament since March, Robredo won his first title of the year at Sopot on Sunday and had reached the quarterfinals at Kitzbuhel prior to that.
Hrbaty, who recorded consecutive wins in an event for the first time since Casablanca in April, will face Hewitt in the third round.
A semifinalist at Wimbledon, Gasquet had a rally turned aside in a 6-3, 6-7 (7-9), 6-4 loss to Spaniard Fernando Verdasco.
It was the Frenchman's first loss in five meetings with the 23-year-old Verdasco, who will face the winner between Canadian Frank Dancevic and American Wayne Odesnik.
Also falling by the wayside was No. 13 Andy Murray, who experienced a setback in coming back a wrist injury that sidelined him for more than two months.
A day after a straight-sets victory over American Robby Ginepri, the Brit Murray struggled serving in a 6-2, 6-2 upset loss to Italian qualifier Fabio Fognini.
Ranked No. 139 in the world, Fognini captured five breaks. He won 77 percent of the points on Murray's second serve (17-of-22).
Fognini next will take on Federer.
A quarterfinalist at Washington last week, No. 12 Tommy Haas advanced when American Michael Russell retired with a back strain. The German was leading, 6-3, 3-0, when play was stopped.
Former top 10 player David Nalbandian upset No. 16 David Ferrer of Spain, 7-6 (7-4), 6-1. It was just the 16th win in 29 matches this year for the Argentine, who has not advanced past a quarterfinal this season after winning three titles over the previous two years.
Nalbandian next will face third seed Novak Djokovic of Serbia.
First prize is $400,000.