By Steve Keating

MONTREAL (Reuters) - Rafael Nadal's tender knees passed their first test when the Spaniard returned to action on Monday in a doubles win at the Montreal Masters.

Sidelined for more than two months with tendonitis in both knees, Nadal looked fighting fit as he teamed up with coach Francisco Roig to beat Serbian duo Novak Djokovic and Dusan Vemic 7-5 6-4.

"It's great to be back and I had a lot of fun playing with Francisco," Nadal told reporters. "A win is a win and doubles is fun and important so I am happy about the way it went today.

"I am really looking forward to playing singles on Wednesday."

It was a positive first step in what is expected to be a testing comeback for the 23-year-old Spaniard, who opens the defense of this Montreal crown with a second round clash against either compatriot David Ferrer or Serb Viktor Troicki.

Playing his first event since being stunned in the fourth round of the French Open on May 31, much has changed for Nadal as rival Roger Federer took his French Open and Wimbledon titles along with the world number one ranking.

Federer's life has also undergone some dramatic changes.

Since capturing a record 15th grand slam singles title at Wimbledon in July, Federer and wife Mirka have become parents to twin girls, the Swiss arriving in Montreal with his family in tow.

BEAMING DAD

The beaming dad said he was relishing his new parental duties, including changing diapers, but on the court the Swiss insisted he would be all business when he begins his U.S. Open build-up.

Federer, who celebrated his 28th birthday last Saturday, opens his Montreal bid on Tuesday against Canadian hope Frederic Niemeyer, who reached the second round with a 7-5 6-1 win over Russian Igor Kunitsyn.

Ranked 487th in the world and the oldest player in the draw, the 33-year-old wildcard also became a father for the first time recently and is counting on Federer feeling the affects of a few sleepless nights before their match.

"I also played a tournament with my daughter after two weeks and I didn't sleep very much that week," said Niemeyer, after notching first tour-level win since March 2008.

"He has two (daughters) so hopefully he sleeps none. Hopefully he's a bit off and I'm playing well."

In other first round action, ninth seed Gilles Simon crushed Canada's Frank Dancevic 6-1 6-2 in a rain-interrupted match to lead a parade of Frenchmen into the second round.

Thirteenth seed Gael Monfils made a winning return from a two-month injury layoff battling past former-champion Marat Safin 6-2 3-6 6-2.

Jeremy Chardy also advanced, rallying past Taiwan's Lu Yen-hsun 6-7 6-3 6-4 to set up a second round meeting with world number three Andy Murray.

Paul-Henri Mathieu outlasted Spain's Guillermo Garcia-Lopez 7-5 0-6 6-1 while Germany's Rainer Schuettler beat Florent Serra to spoil an otherwise perfect day for the French contingent.

Russia's Mikhail Youzhny provided the only upset on opening day, dispatching 14th seeded Croatian Marin Cilic 4-6 7-6 6-1.

(Editing by John O'Brien)