The Serb second seed, a two-time winner in Cincinnati, won his 19th match against the French showman without a defeat.
He needed just 69 minutes to follow top-seeded Carlos Alcaraz into the last eight, the Spaniard withstanding a string of frustrating rain delays to beat Tommy Paul 7-6 (8/6), 6-7 (7/0) 6-3 and avenge a loss to the American last week.
Djokovic applied the pressure early against Monfils in the battle of 36-year-old veterans, denying the number 211 his first last-eight place here since 2011.
Djokovic will next take on American Taylor Fritz, who advanced after Dusan Lajovic retired with a toe injury trailing 5-0.
Djokovic broke once to claim the first set and quickly ran up a 5-0 lead in the second.
He faltered while trying to serve it out, dropping his serve only to complete the contest two games later.
“Gael is one of the most athletic guys on the tour and he showed that in the first set,” Djokovic said. “Every ball comes back from him.
“I served well and made him play. After the first break I didn’t look back and raised my level.
“I had an almost flawless second set,” added Djokovic. “Hopefully I can keep it going.”
Things weren’t quite so easy for Alcaraz against a player who had stunned him in the quarter-finals at Toronto last week.
Alcaraz had squandered three match points against Paul before the first of a string of rain delays that forced players off and on the court in a logistics ballet that finally ended after more than three hours.
The Spaniard held on to book a quarter-final meeting with Australian Max Purcell, a 6-4, 6-2 winner over Swiss Stan Wawrinka.
After the final rain pause, Alcaraz, up a break in the third, came out and finished off the final few points, advancing with 40 winners and 61 unforced errors.
“I really wanted to win after what happened against him last week,” admitted Alcaraz, who reached his fifth Masters 1000 quarter-final this season.
Alcaraz brought out the best of his big game to tame Paul in a contest first interrupted at 4-3 in the deciding set by the weather.
The top seed came back from a break down four times in the first two sets, but he couldn’t put it away when he had three opportunities in the 12th game of the second set.
After the 15-minute game, Paul dominated the tiebreaker.
Alcaraz was up a break and serving at 4-3 in the third when rain stopped play for just over an hour.
Then came a stop-start scenario that saw the court dried and players warmed up only for rain to start again.
“All of the starting and stopping was not easy,” Alcaraz said. “I handled the waiting well.”
There was disappointment however, for third-seeded Daniil Medvedev and fourth-seeded Stefanos Tsitsipas.
Alexander Zverev beat Medvedev 6-4, 5-7, 6-4 in a battle of former champions while Poland’s Hubert Hurkacz beat Tsitsipas 6-3, 6-4.
– Swiatek advances –
In the WTA draw, world number one Iga Swiatek changed her dress after losing the opening set and came back a new woman, defeating China’s Zheng Qinwen 3-6, 6-1, 6-1.
“My coach told me a few weeks ago it might be a good idea to change clothes and reset,” the Pole said. “I tried it and it worked, I got a different vibe in the second set — thank you, coach.”
Wimbledon champion Marketa Vondrousova beat Sloane Stephens 7-5, 6-3 to reach the quarter-finals of an event in which she was 0-3 in prior appearances.
“It has been really special this week. I came here not knowing how I would play,” she said. “It’s such a pleasure to be back as Wimbledon champion.”
Third-seeded Jessica Pegula, last week’s Montreal winner, was defeated in a match which concluded in light drizzle, losing 6-4, 6-0 to Marie Bouzkova.
Fourth seed Elena Rybakina joined number eight Maria Sakkari in exiting.
Rybakina, last year’s Wimbledon champion, retired at 6-4, 2-5 against Italian qualifier Jasmine Paulini while Sakkari went down 6-3, 2-6, 6-3 to Czech Karolina Muchova. Croatia’s Donna Vekic retired with viral illness, handing a 5-2 decision to three-time Grand Slam finalist Ons Jabeur.
Seventh seed Coco Gauff won a battle of teenagers over Czech Linda Noskova 6-4, 6-0 in 62 minutes.
