David Ferrer continued his poor run of form, falling out in the first round of the ATP Western and Southern Open to Julien Benneteau.
Despite levelling in the second set, Benneteau proved too strong on the Cincinnati hard courts, while he was joined in the second round by Jared Donaldson, Kevin Anderson, Pablo Cuevas and Reilly Opelka.
Grigor Dimitrov continued his good hard court form this year, comfortably accounting for Gilles Simon, while Marin Cilic also defeated Victor Troicki in straight sets.
In the final match of the day, Nick Kyrgios also won in two sets, defeating Frenchman Lucas Pouille.
CILIC MAKES LIGHT WORK OF TROICKI
After losing to Gael Monfils in the Rio round of 16, Marin Cilic continued his US Open preparations with a comfortable victory over world number 35 Troicki, emerging victorious 6-3 6-4.
Despite losing to Cilic in the opening round in Rio, Bulgarian Grigor Dimitrov was in fine touch against Gilles Simon, dictating play from the centre of the court in his 6-1 6-3 victory.
After losing to Kei Nishikori at the quarter-final stage in Toronto, the world number 34 Dimitrov will hope to finish the late-season strongly, with topsy-turvy form since making the final in Istanbul.
Fresh off winning in Atlanta, Nick Kyrgios served 23 aces on his way to dispatching Lucas Pouille in an assured 6-2 7-5 win.
Kyrgios did not look troubled on serve, eventually building pressure on the world number 26 to secure the decisive break in the 12th game of the second set and progress.
FERRER FORM SLUMP CONTINUES
Benneteau claimed the biggest scalp of the day in his 6-2 4-6 6-4 win over Ferrer - a week after the Spaniard's disappointing exit at the Rio Olympics.
After Ferrer's loss to world number 81 Evgeny Donskoy, it looked as though the Spaniard might be able to salvage the match against Benneteau.
However, the world number 294 regained his composure, even winning 42 per cent of points returning serve.
Gael Monfils will begin his campaign on Tuesday against Pablo Carreno Busta, while Los Cabos finalists Ivo Karlovic and Feliciano Lopez take on Mischa Zverev and Juan Monaco respectively. Second-seeded Swiss, Stan Wawrinka, also begins when he faces Donaldson, who defeated Nicolas Almagro in straight sets.