Wimbledon runner-up Nick Kyrgios let an early lead slip but regrouped on Tuesday for a 6-4, 6-4 first-round victory over Sebastian Baez at the ATP Montreal Masters.

World number 37 Kyrgios, a title winner last week in Washington, next faces world number one Daniil Medvedev, who won last week's Los Cabos crown.

"It will be a great test and a lot of fun," Kyrgios said. "He's a hell of a player. He's unorthodox – an animal.

"I want to see where my game's at so I'll be giving it my best shot."

The day was a disaster for Britain's Andy Murray as the treble Grand Slam winner was cut off in the first round, a 6-1, 6-3 victim of Taylor Fritz.

The American presented nothing but frustration to Murray, who lost with just seven winners and 19 unforced errors while Fritz fired 30 winners in 87 minutes.

"It's an honour to be on court with Andy," Fritz said. "He has been dominating since before I came onto the tour."

Medvedev's chances of retaining the top ranking over the next month are on the line when he plays Kyrgios.

The 27-year-old Australian secured his seventh straight match victory since losing to Novak Djokovic in last month's Wimbledon final.

Kyrgios said he is on point physically but had to work to maintain mental strength in the match played 48 hours after lifting the trophy in the US capital.

"Physically I feel fine, mentally I'm tired," he said. "I've not had much sleep in the last few days.

"I've put Washington behind me already. I've not played great in Montreal in the past, so I'm just trying to create some good memories for myself."

Kyrgios held a comfortable 5-1 margin in the opening set before Argentine Baez fought back to 4-5 before dropping the set.

The Aussie managed to stop the rot and left nothing to chance in the second set as he closed out victory.

Kyrgios has won 13 of his last 14 singles matches and stands 28-7 this breakthrough season.

Earlier, fellow Aussie Alex De Minaur finished off a match delayed from Monday by rain, winning four of the only five points remaining to add to the misery of Canadian Denis Shapovalov 7-5, 7-6 (7/4).

Shapovalov stands 1-9 since upsetting Rafael Nadal at Rome last May as his slump deepens.

French showman Gael Monfils defeated Spain's Pedro Martinez 7-6 (7/4), 3-6, 6-2.

Matteo Berrettini's comeback from the Covid-19 positive test which forced him to quit Wimbledon stalled out, with the Italian losing 6-3, 6-2 to Paolo Carreno Busta in the opening round.

Berrettini played a clay final last month against Casper Ruud in Gstaad, but was unable to make a decent start to his pre-US Open cement season.