The fifth seed reached the second round over Japan's Taro Daniel with a 6-3, 6-2, 7-6 (1) victory.
Raonic had a smooth ride in the first two sets, but hit speed bumps as he tried to serve it out.
"Maybe just lacked a little bit of intensity," Raonic said after the match. "Where I needed to, I played well.
"It's really hard to gauge my level because of the way things went throughout the match. But I feel like just the way I've prepared for this event, I'm feeling more comfortable than any of the other ones, so that's a good thing."
The Thornhill, Ont., product saved two break points with aces in the 10th game leading 5-4 but hit the net with a backhand to make it 5-5 in the third set. The set went to a tiebreaker where Raonic finally got the job done.
Raonic took just over 90 minutes to earn victory with 20 aces and nearly 60 winners.
"I've been playing my best tennis this year," he said. "I put a lot of focus into this event. I'm glad to be here and start playing.
"I'm hoping to stay for as long as possible."
Vasek Pospisil fell victim to a shoulder injury sustained from practise last week.
The 46th-ranked Vancouver player who won the Wimbledon doubles title, lost to clay-courter Simone Bolelli 2-6, 6-4, 6-2, 3-6, 6-3.
Pospisil revealed that he hurt his shoulder on one serve last Tuesday but added that the problem is slowly healing.
Pospisil fired 13 aces but was weighed down by 51 unforced errors and managed only 21 winners. He won his only match in New York during his 2011 debut after also going out in the first round a year ago.
"I've not been serving much, I lost my rhythm," said the Canadian. "I still have pain in my shoulder and that's never a good thing.
"I couldn't serve like I usually do, that seemed like a big part of the match today. It's only my serve that's affected. But you're then not as confident on serve and not as loose on return games. It changed the whole outlook on the match.
"I'm not stressed about doubles, the shoulder is getting better. Four days ago I could barely serve."
Sharon Fichman of Toronto, Aleksandra Wozniak of Blainville, Que., and Montreal's Francoise Abanda were also eliminated in the women's draw.
Fichman returned from arthroscopic knee surgery to a crushing 6-1, 6-0 loss to fourth-seed Agnieszka Radwanska of Poland in a match that took 47 minutes. Wozniak lost to Japan's up-and-coming Kurumi Nara, 6-2, 6-1, in 59 minutes.
The 17-year-old Abanda, playing in her first match at a Grand Slam, went down to 2013 Wimbledon finalist Sabine Lisicki 6-3, 7-5.
"It was a great feeling to be playing my first Slam," Abanda said. "I had a tough opponent so that was tough, but it was great for me to experience that this year at age 17."
The No. 112-ranked Fichman was playing her first match since losing in the first round of Wimbledon on June 24. She had surgery to repair torn cartilage in her knee in late July, the result of injuries suffered in World Team Tennis.
"Only I know what I've been going through," she said. "Considering all that I have on my plate, I definitely did the best job that I could. I'm proud of my effort. I knew that even on the best of days, Radwanska would not be the easiest match."
Fichman is still searching for her first singles victory at a Grand Slam after first-round losses in New York last year and at Roland Garros and Wimbledon in 2014. She was rusty Monday, with 24 unforced errors and 23 winners from her Radwanska.
"I don't know how long it will take me to come back," said Fichman. "I'm doing the best that could be asked for after the surgery and the ankle and everything. I'll do all that I can, eventually I'll get there. All the struggles and hard work will make me better at the end."
The No. 97-ranked Wozniak suffered through 28 unforced errors as she ended her Grand Slam season without a victory at any of the three majors she played — Paris, London and New York.
The 26-year-old Canadian, who has struggled in her comeback from a shoulder injury several seasons ago, was playing her first elite-level match since losing in the first round of the Rogers Cup.
Montreal's Eugenie Bouchard starts her tournament on Tuesday playing Olga Govortsova of Belarus.