Canada's Felix Auger-Aliassime and Denis Shapovalov dropped their opening singles matches at the U.S. Open on Tuesday.

And on the women's side, Leylah Fernandez of Laval, Quebec, didn't do much better despite being the 23rd seed in the Grand Slam tourney.

But 22-year-old Gabriel Diallo of Montreal opened the evening session in New York City with a brilliant four-set upset over Jaume Munar of Spain in a match that took three hours, 23 minutes to play.

Diallo defeated Munar 6-4, 3-6, 6-3, 7-5, recording 20 aces along the way. Diallo had 52 unforced errors compared to Munar's 15, but the Canadian dominated in winners 39-15.

Czechia's Jakub Mensik posted a 6-2, 6-4, 6-2 win over the 19th-seeded Auger-Aliassime in first-round play while Shapovalov dropped a 6-4, 7-5, 6-4 decision to Botic van de Zandschulp of the Netherlands.

WATCH | Auger-Aliassime knocked out of U.S. Open in 3 sets:

Felix Auger-Aliassime flames out of U.S. Open

Montreal's Felix Auger-Aliassime was upset by 18-year-old Jakub Mensik of the Czech Republic 6-2, 6-4, 6-2 in the first round of the U.S. Open.

Mensik scored his sixth break in the final game of the match, with Auger-Aliassime committing his 36th unforced error on match point.

Auger-Aliassime, from Montreal, has had a string of tough results since finishing fourth in men's singles and teaming with Gabriela Dabrowski to win mixed-doubles bronze at the Paris Olympics.

He was drummed out in the first round of his hometown tournament at the men's National Bank Open.

Auger-Aliassime also fell in the third round of the Cincinnati Open when his British opponent, Jack Draper, was given the call on match point despite the ball seeming to hit the ground on his side of the court before going over the net.

Shapovalov, a former top-10 player from Richmond Hill, Ont., is currently ranked No. 105 in the world.

WATCH | Shapovalov eliminated from final major of season:

Shapovalov eliminated in opening round at US Open

Denis Shapovalov of Richmond Hill, Ont., fell to Botic van de Zandschulp of Netherlands 6-4, 7-5, 6-4 in straight sets in first round action at the US Open in New York Tuesday.

Fernandez, the 2021 U.S. Open finalist, jumped out to an early lead against Russia's Anastasia Potapova, easily winning the first set 6-2. But the Russian rallied to win the second set 6-4 and won the deciding set 7-5.

Both players had six double faults, while Fernandez had 19 of the 47 unforced errors in the two-hour, 26-minute match.

In other women's action, 2019 champion Bianca Andreescu of Mississauga, Ont., was to face fifth-seed Jasmine Paolini of Italy in the evening draw.

WATCH | Fernandez goes down in 3 sets:

Fernandez becomes 3rd Canadian Tuesday to fall at US Open

Leylah Fernandez of Canada was defeated in opening round action 2-6, 6-4, 7-5 by Anastasia Potapova on day two play at the 2024 US Open in New York.

Sinner wins 1st match since doping case

If there were any boos directed Jannik Sinner's way at the U.S. Open, they were imperceptible amid the polite applause from the sparse crowd at Arthur Ashe Stadium when he was introduced before the No. 1-ranked man's first match since his doping case emerged a week ago.

After a sloppy and slow opening set, Sinner pulled away for a 2-6, 6-2, 6-1, 6-2 victory against Mackie McDonald of the United States to reach the second round at Flushing Meadows.

"The response from fans, I feel it has been great. Also throughout, when the news came out, in the practice sessions, there was a lot of support, which I'm very glad and happy about," Sinner said. "It is still a little bit not easy. You have to go through, day by day."

There were no apparent signs in the stands or disparaging shouts making reference to what no one knew about for months: Sinner tested positive twice for trace amounts of the anabolic steroid Clostebol in an eight-day span in March.

Nothing was announced publicly until last week, when word came that the 23-year-old Italian was docked prize money and ranking points from the tournament where the first result appeared, but he escaped a suspension because it was ruled he was not at fault and the drug entered his system unintentionally, through a massage from his physiotherapist.

In his first public comments on the matter, at a pre-tournament news conference Friday, Sinner said he had fired the fitness trainer who bought an over-the-counter spray containing Clostebol in Italy and the physiotherapist who used it for a cut on his finger before treating the player and transferring the substance to his body.

Against McDonald, an American ranked 140th who fell to 4-13 in 2024, Sinner was way off the mark at the outset. He double-faulted. He put what should have been an easy smash into the net. He messed up volleys. It added up to 14 unforced errors in the initial set, helping McDonald grab five games in a row from 2-all to collect that set and go up a break in the second.

No. 1 Swiatek pushed in 1st-round victory

Top-ranked Iga Swiatek fought through a tough first-round match at the U.S. Open, edging Kamilla Rakhimova 6-4, 7-6 (6).

The 2022 champion was one point away from being forced to a third set when Rakhimova took a 6-3 lead in the tiebreaker. But Swiatek recovered and won the next five points, finishing the match when her opponent missed a backhand.

Swiatek improved to 22-1 in first-round matches at Grand Slams, including 6-0 at the U.S. Open.

Jannik Sinner, the No. 1 seed on the men's side, followed Swiatek onto Arthur Ashe Stadium for his opening-round match. It was his first action since word emerged that he had failed two drug tests in March but escaped suspension.

No. 7 Hubert Hurkacz and No. 25 Jack Draper moved into the second round.

Osaka makes quick work of Ostapenko

Two-time champion Naomi Osaka made an impressive return to the tournament by beating No. 10 Jelena Ostapenko 6-3, 6-2.

Osaka, the 2018 and 2020 champion in Flushing Meadows, needed only 64 minutes to oust Ostapenko, the 2017 French Open champion. It was the kind of performance that was standard from Osaka when she reached No. 1 in the rankings and won four Grand Slam titles between 2018-21.

But she had missed large gaps of time since, first for mental health breaks and then after giving birth, and just this month posted on social media about the struggles of not playing as well as she used to after failing to qualify for the tournament in Cincinnati.

She smashed 19 winners against only five unforced errors Tuesday.

Elsewhere, Danielle Collins's singles career at Grand Slams ended when the No. 11 seed was upset by fellow American Caroline Dolehide 1-6, 7-5, 6-4. Collins, 30, has announced she plans to retire at the end of the season. She is still entered in doubles at Flushing Meadows.

Evans beats Khachanov in longest U.S. Open match ever

Also, Dan Evans won the longest match at the U.S. Open since tiebreakers were introduced in 1970, beating Karen Khachanov 6-7 (6), 7-6 (2), 7-6 (4), 4-6, 6-4 in five hours, 35 minutes.

Evans trailed 4-0 in the fifth set before running off the final six games. The final point, fittingly, was a marathon 22-shot rally, with Evans on the defensive for much of the point before hitting a hard shot to the corner that the No. 23-seeded Khachanov couldn't get back over the net with his backhand.

The previous record was five hours, 26 minutes, when Stefan Edberg beat Michael Chang in a five-setter in the 1992 semifinals.

Evans improved to 5-0 against Khachanov, a semifinalist at the 2022 U.S. Open, by emerging in a match in which the sets lasted 68, 67, 72, 67 and 61 minutes.

The British player who played doubles with Andy Murray at the Olympics in the three-time Grand Slam champion's final tournament finished with a 201-191 edge in total points. He advanced to play Mariano Navone of Argentina in the second round.