Novak Djokovic and Andy Murray won their opening matches at the Western & Southern Open in straight sets.
World number two Djokovic defeated Andreas Seppi of Italy 7-6 (7/4) 6-2 on a court baked in sunshine and 86-degree heat.
The Serbian said afterwards: "Sometimes it's really hard to try to stay fit for every single tournament and try to perform your best.
"In the last three, four weeks, I have changed three different cities, places, surfaces, conditions. I'm still trying to figure out where I am."
Defending champion Murray, the world number four, defeated Sam Querrey 6-2 6-4.
The Olympic gold medallist had to withdraw from the Rogers Cup because of a sore knee, which he blamed on the transition from grass to hard courts, but allayed fitness fears after his defeat of the American.
The Scot said: "The knee felt fine. I moved well today.
"It still was giving me a little bit of trouble in practice for a couple of days before the tournament, but it felt fine, much better on the court today.
"I moved well, so I'm hoping it won't be a problem."
Elsewhere, 10th seed Mardy Fish advanced to the third round by defeating Argentina's Carlos Berlocq 6-3 6-1, Serbian Victor Troicki progressed with a 6-2 6-0 win over Australian Lleyton Hewitt, while Russia's Nikolay Davydenko beat Germany's Florian Meyer 6-3 6-3.
In the women's event, Agnieszka Radwanska eased into the third round with a 6-4 6-3 win over Sofia Arvidsson.
The top-seeded Pole defeated Arvidsson in one hour and 23 minutes in a match blighted by poor serving - both players finished the match with a first-serve percentage under 60.
Czech fourth seed Petra Kvitova came from a set down against Germany's Mona Barthel to win 3-6 6-2 7-5 and advance to the third round in two hours and eight minutes.
Sara Errani, the seventh seed from Italy, saw off Slovakia's Daniela Hantuchova 6-4 6-4, while Russia's Anastasia Pavlyuchenkova defeated Germany's Julia Goerges 6-3 6-0.
The string of retirements in the women's draw continued with 11th seed Dominika Cibulkova pulling out while trailing 7-6 (7-1) 4-1 against Czech qualifier Andrea Hlavackova.
Yaroslava Shvedova conceded her match against Urszula Radwanska despite taking the first set, although the Kazakh was trailing 4-1 in the second.
Elsewhere, Sloane Stephens won the battle of the wildcards with a 6-2 6-1 defeat of Camila Giorgi, Venus Williams overcame Chanelle Scheepers 2-6 6-3 6-2, Ekaterina Makarova beat Anna Tatishvili 6-1 6-2, Caroline Wozniacki downed Sesil Karatantcheva 6-1 6-0 and Shuai Peng won 6-3 6-4 against Roberta Vinci.
Top seed Roger Federer recorded a 6-3 6-2 win over Alex Bogomolov Junior to set up a last-16 clash against Bernard Tomic.
The top-ranked Swiss must make the final in Cincinnati to ensure Djokovic does not regain the world number one ranking.
Fourth seed David Ferrer was a shock departure, losing 6-4 6-1 to Stanislas Wawrinka, while seventh seed Janko Tipsarevic and Juan Monaco were also third round casualties.
Serb Tipsarevic retired due to heat exhaustion while trailing Spain's Pablo Andujar 6-4 4-1, while Argentina's Monaco was defeated by Czech Radek Stepanek 7-6 (7-2) 6-1.
Sixth seed Juan Martin del Potro ended German veteran Tommy Haas' tournament with a 7-5 6-2 win, while Canada's Milos Raonic was tested before surpassing Cyprus' Marcos Baghdatis 6-7 (6-8) 6-3 6-4.
Kei Nishikori, the 14th seed from Japan, dismissed American wildcard James Blake 2-6 6-4 6-4, and French lucky loser Jeremy Chardy set up a last-16 clash against Murray with a 6-4 6-3 win over Uzbekistan's Denis Istomin.