Simona Halep earned her first victory since last May's calf injury pause as the three-time finalist defeated Magda Linette 6-4, 3-6, 6-1 on Tuesday at the ATP-WTA Cincinnati Masters.
American teenager Coco Gauff lined up a contest with second seed Naomi Osaka through a 6-1, 6-2 hammering of Hsieh Su-Wei.
On a day when rain forced a reorganisation of the afternoon and evening schedule, women's top seed Ashleigh Barty and Olympic gold medalist Alexander Zverev never made it onto court.
Halep only returned to the Tour last week in Montreal, losing in the opening round. She remained confident that progress was not far away after her injury in Rome three months ago.
The Romanian defeated her Polish opponent for a second time, taking just over two hours.
"It was a strong match. In my opinion, both of us played really well," the 29-year-old said. "Maybe the rain delay helped me a little bit to find energy. I was a little bit stronger physically and mentally in the third set.
"It means a lot, I'm really happy to win. It's always important to have victories when you come back from an injury.
"This was the first time in my life that I've had this kind of problem, and I didn't really know how to manage it. But I'm happy to be back, happy to play pain-free. I'm just trying to build my confidence back."
The 12th seed, a former number one, played Cincinnati finals in 2015, 2017 and 2018. She dominated her 44th-ranked opponent with 11 aces and four breaks of serve.
Gauff won her last match with four-time Grand Slam champion Osaka at the Australian Open last year. Gauff, 1-1 in the series, will play Osaka for the first time outside of a Grand Slam
"I've learned (from their past matches) to just focus on me and not on the expectations of others," she said of the high-profile meeting with the former number one.
Montreal champion Camila Giorgi fell 6-2, 6-2 in her opening match to Jessica Pegula in a re-run of the pair's semi-final last Saturday in Canada.
Hopes ended for Britain's Johanna Konta in a 3-6, 7-6 (7/5), 6-2 loss to Czech Karolina Muchova.
Konta has now lost in the Cincinnati first round in three of her last four appearances at the tournament, reaching the semi-finals in 2020 when the event was staged in New York under strict Covid-19 protocols.
In the men's draw, Wimbledon finalist Matteo Berrettini returned from a thigh injury layoff but only just managed a 6-7 (5-7), 6-3, 7-5 defeat of Spaniard Albert Ramos-Vinolas.
The Italian fifth seed showed rust after last playing the All England Club final against Novak Djokovic,
But he prevailed after saving five break points in the final set to end it on a drop shot winner on match point.
"I knew it would be tough, but I'm really happy - as much with the mindset as with the tennis," Berrettini said.
Last week's Toronto quarter-finalist Gael Monfils was untroubled by Serb Dusan Lajovic in a 7-6(7/0), 6-2 win in 91 minutes as the Frenchman overcame nine double-faults.
Grigor Dimitrov advanced to the second round as he eliminated 13th seed Roberto Bautista Agut 6-3, 6-4. American Sebastian Korda won his 23rd match this season as he quickly brushed aside Serb Laslo Djere 6-4, 6-4 to line up against second seed Stefanos Tsitsipas.
Weekend Toronto semi-finalist John Isner was bothered by back pain throughout but relied on his huge serve to wrap up a 4-6, 7-6 (7/5), 6-4 comeback over Brit Cameron Norrie.
Isner finished off with his 26th ace as Norrie lost in a first round for the second week in succession.
The 36-year-old American moved into the second round to play Italy's 11th seed Jannick Sinner