In the upset of the day, Mirza Basic of Bosnia, a lucky loser from qualifying playing his first main-draw match on the ATP Tour, downed seventh-seeded Jerzy Janowicz of Poland 7-6 (5), 3-6, 7-6 (5).
The 21-year-old Basic earned a match point with a drop volley and won the match when Janowicz reached another drop shot but sent it back long.
Basic was playing only because Philipp Petzschner withdrew with a shoulder injury.
"They only told me at noon that I would be playing. I was a little scared, I was a little nervous, I was playing a big server on grass, on centre court," Basic said.
Monfils broke Raonic for 2-1 in the first set and never looked back. The Frenchman also went up a break at the start of the second set. Raonic, from Thornhill, Ont., put some pressure on Monfils in the final game, but Monfils hit a drop shot followed by a lob, and served out the match with an ace.
Once ranked as high as No. 7 in the world, Monfils missed several months in 2012 because of a right-knee injury that also cut his season short. He was ranked No. 119 a month ago, but has jumped to No. 67 this week. Monfils wasted four match points in losing to Tommy Robredo in the third round of the French Open.
"I am just happy to be on court, enjoying every second," said Monfils, who joked with line officials during the match and threw his racket while trying to reach a smash from Raonic.
Monfils said adjusting to grass from clay in Paris was not easy.
"It's very hard for my movement," Monfils said, adding that he'd rather lose a point than risk re-injuring his knee. "I am more mature after what I went through last year. I've grown up. I try to do simple things."
In other first-round matches, sixth-seeded Philipp Kohlschreiber of Germany cruised past Carlos Berlocq of Argentina 6-3, 6-1, Ernests Gulbis of Latvia beat Marcos Baghdatis of Cyprus 6-4, 3-6, 6-3, Leonardo Mayer of Argentina defeated German wild card Jan-Lennard Struff 7-6 (4), 6-4 and Jurgen Melzer of Austria outlasted qualifier Riccardo Ghedin of Italy 7-6 (5), 3-6, 6-4.
Basic played four matches on grass as a junior at Wimbledon six years ago, and none again until three rounds in qualifying in Halle. He was No. 5 in junior ranks but the transition to the pro tour has been slow — he is ranked No. 240.
Basic began playing tennis in his native Sarajevo "because there was nothing else to do, it was after the war and everything was destroyed."
Splitting time between Sarajevo and a tennis academy in Brussels, Basic plays league tennis in various countries while he tries to stay afloat financially.
Basic took a 3-1 lead in the final set, but was broken back immediately by the 22nd-ranked Janowicz, who finished with 16 aces.
"I didn't think I would lose my serve so easily so I lost some confidence. I was trying to hold my serve and wait for the tiebreaker, I knew that was my chance," said Basic, who served 10 aces.