Federer, who won the title twice in 2004 and 2006 in Toronto, lost in the final in Montreal last year to Alexander Zverev and was previously on the entry list for this years tournament, though has made the fairly unsurprising move to withdraw.
The Swiss master is scheduled to play in Cincinnati, where he has won a tournament-record seven times but missed it a year ago because of a back injury. The 20-time Grand Slam champion added that he was "sorry" to miss the tournament and wished it every success.
The US Open in NY starts two weeks later.
The world No. 2 from Switzerland lost against Germany's Alexander Zverev in the final of the event previous year in Montreal.
Federer is the first top-10 player to drop out of THE ATP World Tour Masters 1000 event, August 4-12.
The Rogers Cup will now feature nine of the top 10 players on the ATP World Tour.
Roger Federer said he felt'awful after his Wimbledon defeat to Kevin Anderson
Federer's decision comes as a mild surprise. I don't know what's going to happen.
The 36-year old Federer has not played since falling to Kevin Anderson in the quarterfinals at the All England Club in an epic five-setter.
"I think it'll be hard for him away from grass courts", Henman said.
"I'm looking forward to also go out there and play my best and see where it takes me", the Serbian former world No 1 said. He has won 35 matches overall in Canada and performed in very close finals against Novak Djokovic, Andy Murray and Jo-Wilfried Tsonga.
The British player has dropped below the top 800 in the men's singles rankings.
Two Canadians make Booker long list
Sales of the selected books can increase hugely with a nomination, while a win could increase sales by 1,000%, according to various estimates.
Boeing’s already-delayed Starliner spacecraft suffers propellant leak during engine testing
Updates on commercial crew test schedules for both Boeing and SpaceX are expected to be released as soon as next week. The June hot-fire test at NASA's White Sands Test Facility in New Mexico didn't go entirely as planned, though.
Flash flood watch extended through Wednesday for Williamsburg region | Williamsburg
WEEKEND OUTLOOK: Some small shower and thunderstorm chances remain for the weekend, but overall it should be much quieter. As the rain continues to add up, the threat for flooding will increase for at least the next day or two.