Alexander Zverev defeated fellow former Cincinnati Open champion Grigor Dimitrov 6-2, 6-2 on Tuesday to advance to the second round.
Germany's Zverev, the 16th seed, claimed the title at his last participation in 2021 while Dimitrov lifted the trophy in 2017.
A year ago, Zverev was healing from ankle surgery following a serious injury at Roland Garros.
The 25-year-old is hoping to turbocharge his hardcourt game after falling in the Toronto second round last week, with the US Open starting on August 28.
"Last week was really bad for me," Zverev said. "I just played poor tennis. Today I was patient and adjusted a bit better.
"Grigor has a big serve and is dangerous. He can play very aggressively.
"You have to be there (in the match) from start to finish and focus on yourself. That's what I did and I'm happy."
Zverev was joined in the second round by former semi-finalist Stan Wawrinka, who polished off a win after rain called a halt to his match the night before.
Switzerland's three-time Grand Slam champion defeated American Brandon Nakashima 6-3, 6-7 (5/7), 6-3, requiring just two concluding games to reach the second round after the overnight pause.
"It was a tough day yesterday, a lot of rain, a lot of interruptions," the 38-year-old said.
"I was playing well. It was easy today to focus from the first point, to get into the match directly.
"I hope I can keep pushing it further," added Wawrinka, who now takes on another American in 10th seed Frances Tiafoe.
Borna Coric concluded unfinished business with Petra Korda, winning 7-6 (7/5), 6-4 while Ugo Humbert defeated fellow Frenchman Arthur Fils 6-4, 3-6, 6-2.
- Wozniacki out -
American ninth seed Taylor Fritz earned a 7-6 (16/14), 6-2 defeat of Czech Jiri Lehecka while Toronto semi-finalist Tommy Paul put out Serb Miomir Kecmanovic 7-5, 7-6 (7/2).
Lucky loser Daniel Altmaier, who replaced Andy Murray when the Scot withdrew with an abdominal injury, fell 6-7 (5/7), 6-4, 6-4 to Australian Alexei Popyrin.
In the WTA draw, the continuing career comeback of former number one Carolina Wozniacki failed to ignite as the Dane went down to French lucky loser Varvara Gracheva 6-4, 6-4 in 96 minutes.
Wozniacki, 33, won her first comeback match a week ago in Montreal before losing in the second round to Wimbledon winner Marketa Vondrousova.
"I didn't play very well and never found my rhythm," Wozniacki said after Tuesday's loss.
"I tried to fight but it was one break in each set. I just didn't return well enough either... I had nine breakpoints and won none of them.
"That's just not good enough when you're playing against players on this level; we'll have to work on that."
But the mother of two is not giving up hope for her comeback effort.
"I still have about 10 days to prepare for the US Open. I feel like I need to do better," she said.
"I'll stay here for a little while and get a bunch of tennis in and put some hours in."
Two-time winner Victoria Azarenka rallied from 1-4 down in the second set to overhaul Barbora Krejcikova 6-3, 7-5.
Azarenka's fellow double Grand Slam champion Petra Kvitova took down Anna Blinkova 7-6 (7/2), 6-0.
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