Top seed and defending champion Rafael Nadal overcame a slow start to beat British qualifier Dan Evans 7-6(6), 6-4 on Wednesday and reach the third round of the Rogers Cup in Montreal.
Nadal, playing his first match since losing to Roger Federer in the Wimbledon semi-finals, fell behind 5-2 in the tiebreak before roaring back to prevail in his first career meeting with world number 53 Evans.
The Spaniard jumped to a 2-0 lead in the second set before the players were forced off court for a rain delay and when play resumed Evans got back on serve at 3-3 before Nadal regained his confidence and proved too strong.
The French Open champion said he did not make an ideal start as he struggled to adjust to a new playing surface given the amount of time he logged on clay and grass in recent months.
"After Wimbledon always the mind goes down a little bit because I have been playing long claycourt season, then grass, so you didn't relax," Nadal told reporters. "Then when you finish Wimbledon, it's like your body lose a little bit that tension, so it needs little bit of time to recover.
"Today, as I said before, the main thing is win. I know
with victories that other things should be coming slowly."
Up next for Nadal, a four-times champion in Canada who has never successfully defended a hardcourt title in his career, will be Argentine Guido Pella, who was a 6-3, 2-6, 7-6(2) winner over Moldova's Radu Albot.
In other second-round action, Frenchman Richard Gasquet upset Japanese fifth seed Kei Nishikori 6-7(6), 6-2, 7-6(4) and Croatian 14th seed Marin Cilic was a 6-3, 6-4 winner over Australian lucky loser John Millman.
Hubert Hurkacz defeated world number five Tsitsipas 6-4, 3-6, 6-3 for the Pole's fourth win over a top 10 opponent this year.
The crucial moment came with Tsitsipas facing a break point and trailing 4-3 in the third set.
Hurkacz hit a passing shot that got past the charging Tsitsipas but the Greek believed the shot was wide.
Tsitsipas was angry that he could not challenge the call since there was no Hawkeye review system available to the players on the outside court.
"It's a joke! It's a Masters 1000 event!" Tsitsipas yelled at the chair umpire.
Isner failed to convert any of his break point opportunities and his usually deadly serve didn't quite fire as he fell 6-3, 6-4 to Cristian Garin of Chile.
Garin will face Daniil Medvedev, who crushed Kyle Edmund 6-3, 6-0 in their second-round match earlier in the day.
An all-Canadian battle between Felix Auger-Aliassime and Milos Raonic ended with a whimper when the big-serving Raonic was forced to retire from the match with an apparent back issue ahead of the third set.
Second seeded Dominic Thiem, sixth seeded Karen Khachanov and Frenchman Gael Monfils all advanced to the round of 16 on Wednesday.
Italian seventh seed Fabio Fognini capped the evening session with a 7-6(3), 6-3 win over American qualifier Tommy Paul.
Serena storms into third round in Toronto
Serena Williams made a solid start to her first US Open tune-up with a 6-3, 6-3 victory over Elise Mertens on Wednesday while Naomi Osaka took a step closer to regaining her world No. 1 ranking by reaching the last 16 of the Rogers Cup.
Three-times champion Williams, playing her first match since falling to Simona Halep in the Wimbledon final, married her trademark power with quick movement and a steady stream of booming serves to overwhelm Belgian Mertens.
"It feels good," Williams said of her return to the hard-court surface.
"Definitely feels different as I didn't play a ton this year on hardcourts but it feels good to be back out here."
Williams is keen to get more hard-court matches in before going for a record-equalling 24th career Grand Slam title at the August 26-September 8 US Open and she took some time to hit her stride on Wednesday.
After a pedestrian start, Williams raised the intensity during a lengthy four-deuce service game in the first set that she closed out with a thundering ace to go ahead 4-3 and set the tone for the rest of the 75-minute match.
She went on to win six of the final seven games to seal the win.
Williams will face Russian qualifier Ekaterina Alexandrova in the last 16 and a win would set up a potential quarter-final with US Open champion Naomi Osaka, which would be their first encounter since last year's controversial New York final.
Earlier on Tuesday, Japanese second seed Osaka won the first set against Tatjana Maria 6-2 and advanced when the German qualifier retired.
"I thought I played really well despite the fact that we only played one set," said Osaka. "I'm just happy that it was my first match in a while and I was able to get in the groove of things quickly."
By reaching the last 16, Osaka is set to unseat Australian Ash Barty, who lost her second-round match on Tuesday, as world number one next week, though Karolina Pliskova also has a chance to reclaim top spot if she reaches the semi-finals.
The Czech third seed beat American qualifier Alison Riske 6-4, 6-7 (4), 6-2 and will next face Estonian Anett Kontaveit, who was beating Carla Suarez Navarro 7-5 3-1 when the Spaniard retired with a hip injury.
Fourth seed Halep survived a stern test from American qualifier Jennifer Brady with a 4-6, 7-5, 7-6 (5) victory to reach the last 16 and keep alive her hopes of winning a second consecutive Rogers Cup title.
Halep, playing her first match since Wimbledon, needed a medical timeout to get treatment on her left leg and even after building a 4-0 cushion in the deciding set had to dig deep after Brady mounted a comeback.
"I was tired in the third set, I felt the Achilles since the first point of the match, but it's normal after a break and also coming on the hard courts," said Halep.
"It's very difficult for the ankles and also for the knees."
Next up for Halep will be Russian wildcard Svetlana Kuznetsova, who got by Croatia's Donna Vekic 7-6 (4), 6-3.
Former world number one Caroline Wozniacki fell 1-6, 6-3, 6-4 to Polish qualifier Iga Swiatek, who smacked 34 winners en route to the win over the 2010 champion under the lights.
"It's amazing. I'm feeling great," the teenager said.
"I mean, I'm really tired, and I should be sleeping right now," she said.
Swiatek will need all the rest she can get before she faces Osaka in the third round on Thursday.
Among the other players advancing were Ukrainian sixth seed and 2017 champion Elina Svitolina, Ukrainian teenager Dayana Yastremska, Latvian Jelena Ostapenko, Canadian Bianca Andreescu and Swiss Belinda Bencic.