Cincinnati Masters top seeds Jannik Sinner and Iga Swiatek both secured opening triumphs Wednesday over qualifiers, with Swiatek forced to escape an attempted second-round ambush by France's Varvara Gracheva.
WTA world number one Swiatek needed six match points and more than two hours to escape with a 6-0, 6-7 (8/10), 6-2 victory to start the last major event prior to the US Open.
ATP number one Sinner fought off eight break points as the top seed began the Cincinnati Open with a 6-4, 7-5 triumph over US qualifier Alex Michelsen.
Swiatek ran away with the opening set in 22 minutes but could not close out when leading 5-2 in the second.
The 62nd-ranked Gracheva saved five match points and converted on her own fifth set point to take the match into a third set.
Swiatek regained her poise to finally pull out the unexpectedly tight win to reach the third round.
"It was not easy to finish in the second set," Swiatek said. "She started playing with freedom, with nothing to lose.
"I backed down a bit and she used her chances. I'm happy I got more proactive in the third set and played more intense."
Swiatek said she kept her self-belief.
"I knew I had the game to win it -- I did have five match points," she said. "It was my fault that I didn't close it out, but that happens.
"I was able to play the third like I did at the start of the match. I don't want any regrets. I'm only looking forward. I'm in a peaceful place and I'm playing without expectations."
Italy's Sinner, who turns 23 on Friday, has never been past the third round at the American Midwest venue but is hoping to improve on that showing this week.
Victory in just under two hours marked the 25th on hardcourt this season for Sinner, who was the first man into the third round.
The top seed has not lost before the quarter-final stage of any event since Masters 1000 Shanghai last October.
Teenaged opponent Michelsen went down with 42 unforced errors, his winner count of 14 exactly half of Sinner's total.
"It was a tough match," Sinner said. "I needed to start feeling the conditions and getting used to them.
"I'm very happy to be into the next round. I've struggled here in past years. But today I stayed positive and that helped in the win."
Sinner, who missed the Paris Olympics with tonsillitis and went out last week in the Montreal quarter-finals, said he was not at his best.
"I'm not yet at 100%, but the main goal is the US Open," he said. "I have one week (after Cincinnati) to prepare in the best possible way.
"The matches I can play here will help at the Open."
Sinner next plays Australian Jordan Thompson, a 6-2, 6-4 winner over Sebastian Baez.
- Medvedev eliminated -
Former world number one Daniil Medvedev was ousted in his second-round match, the fourth seed falling to 22-year-old Czech Jiri Lehecka 7-6 (7/2), 6-4. Medvedev also fell in the first round last week at Montreal.
Ninth seed Stefano Tsitsipas played for the first time since splitting last week with his father as coach, with the Greek turning the tables in dramatic fashion to reach the second round 4-6, 6-4, 6-3 over Jan-Lennard Struff.
Norwegian seventh seed Casper Ruud will next face Felix Auger-Aliassime after his 6-3, 6-1 win over US qualifier Aleksandar Kovacevic.
Monday's Montreal champion, Australian Alexei Popyrin, lost his opening match to France's Gael Monfils 7-5, 6-3.
Dane Holger Rune beat Matteo Berrettini 2-6, 6-1, 6-4, ending the Italian's 10-match win streak that included clay titles in Gstaad and Kitzbuehel.
Briton Jack Draper worked for more than two and a half hours to stop Spain's Jaume Munar 7-5, 3-6, 7-6 (7/5).
Paula Badosa joined boyfriend Tsitsipas in the second round after defeating Cincinnati local Peyton Stearns 6-2, 7-5.
Washington winner Badosa next plays 13th seed Anna Kalinskaya, a 1-6, 6-2, 6-3 comeback winner against Katerina Siniakova.