The Canadian pulled out of his Paris Masters semi-final against Andy Murray with a right thigh injury earlier this month and it was touch and go whether he recovered in time for the London year-ender.
Wimbledon runner-up Raonic made a flying start, though, with a 6-3 6-4 Ivan Lendl Group victory over flamboyant Frenchman Monfils.
It was his first victory at the O2 after his debut two years ago resulted in two defeats before he withdrew injured.
Raonic offered up no break points and pummelled the ball with his hefty groundstrokes to keep Monfils on the run.
The world number four, standing 1.96 metres tall and weighing in at nearly 100kg, moved surprisingly smoothly although he said later that there were still some concerns.
"Maybe I hesitated a little bit a few times especially on the longer runs," Raonic told reporters.
"I hope I can get rid of those hesitations as the days go on. I feel like this is an ideal way to start considering the doubts that I was having.
"Some of the things I heard in the diagnosis were not the most positive so this was a great way to turn it around."
Providing Raonic is 100 percent fit, the 25-year-old will be a formidable opponent for defending champion Novak Djokovic when they clash on Tuesday.
He is unlikely to run out of energy either thanks to the dried fruit snacks he was nibbling during the changeovers.
"They were dates," said Raonic. "I don't like them but apparently they are very good for sustaining energy. You can see from how small a bit I take that I really don't enjoy them."
(Editing by Tony Jimenez)
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