The Italian, unbeaten this year, showed the imperious form that took him to victory in Melbourne from the start, racing into a 3-0 first-set lead.
Wildcard Monfils entertained the crowd with some trademark retrieving and the occasional trick shot but initially had no answer to Sinner's consistency, losing the first set 6-3.
But the Frenchman, now 70th in the world, rolled back the years in the second set, delighting the Rotterdam crowd that saw him win back-to-back titles in 2019 and 2020.
Cleverly mixing blistering forehands and well-judged drop shots, he disturbed Sinner's rhythm and broke serve early in the second set, hanging on to close it out 6-3.
A poor Monfils first service game in the deciding set, riddled with unforced errors and a double-fault, handed the initiative back to the Italian, who again opened up a 3-0 advantage.
Sinner's relentless pressure told and the world number four eventually broke the French resistance, finishing off the third set 6-3.
"It's always tough to play against him. He won two times here, so he knows how to play here," said Sinner.
"I'm very happy. I was struggling a bit, I didn't have so much rhythm."
"But in these kind of matches you learn a lot. It gives me confidence for the next one," against Canada's Milos Raonic.
'Biggest win'
Earlier in the day, rising star Holger Rune suffered a shock defeat to Kazakh Alexander Shevchenko and local hero Tallon Griekspoor beat fourth-seed Hubert Hurkacz in a thriller with the help of raucous crowd support.
Both winners said the victories were their "biggest" or "best".
Shevchenko, the world number 57 claimed only his second victory against a top-10 player to set up a quarter-final clash against Bulgaria's Grigor Dimitrov.
In a topsy-turvy match, Shevchenko won a tight first set 6-4 before capitulating 6-1 in the second. The deciding set went on serve until 4-3 before the Kazakh broke the Rune serve.
Shevchenko let out a triumphant scream as he held serve to win the match and afterwards told reporters that he considered this a breakthrough win against the world number seven and third seed.
"It's the biggest win of my career. I'm really proud of myself," he said.
"Already last year, I played some good tennis but I needed the experience of playing top guys like (Hubert) Hurkacz, Rune, (Daniil) Medvedev, (Jannik) Sinner. Now I got it and I'm trying to find a way day by day to play them."
He predicted a very different challenge against Dimitrov, who tends to employ more slice than Denmark's Rune, who relies on a hard serve and powerful forehand.
"I don't really know how you can prepare for Grigor. He's playing some insane tennis at the moment," he said.
Shevchenko is now setting his sights on regularly getting to the business end of top tournaments and believes he has made changes in his game and his preparation to allow him to do that.
"Something clicked. I started to become more mature, more professional," he said.
Griekspoor, the world number 29, showed his battling qualities, coming through 7-6(5), 6-7(5), 7-6(4).
"I'm unbelievably happy. It's the best win of my life," he said.
"This is the reason I play tennis, to play these kinds of matches... I'm really proud of what I did. It shows what I can do," added the 27-year-old Dutchman.
