Britain's Andy Murray said he needs to improve by "10 or 15%" to turn defeats like his first-round loss to Lucas Pouille in Cincinnati into victories.
Murray, 31, in his fourth event after nearly a year out with a hip injury, lost 6-1 1-6 6-4 to the 24-year-old.
"He's a top-20 player," Murray said of the Frenchman, who is ranked 17.
"I only lost four more points in the match than him. It was tight. If I can improve by 10 or 15%, you turn a lot of those matches around."
Murray made six double faults on his way to being broken three times in a one-sided first set.
The Scot recovered to force a decider, but Pouille prevailed to secure his first professional victory over Murray in five meetings.
Two weeks ago Murray pulled out of his Washington Open quarter-final after only finishing his last-16 win at 03:02 local time earlier the same day.
Murray also withdrew from last week's Rogers Cup in Toronto to continue his recovery.
The former world number one looked rusty in the opening exchanges, with two double faults in his first service game setting the tone for a poor opening set.
Murray hit back strongly at the start of the second set, breaking Pouille three times and exerting greater authority on his own serve to force a decider.
Both players showed signs of tension in the opening game of the third set, which lasted nearly 12 minutes.
Murray eventually surrendered the break having led 40-15, with the three-time Grand Slam champion evidently frustrated after double-faulting to hand Pouille the initiative.
The Scot remained in the contest and forced a break point in the fourth game of the set after coming out on top of a memorable rally at 15-15.
However, Pouille held on to maintain his advantage and then served out the match comfortably.
Elsewhere in the first round, former world number one Novak Djokovic eventually saw off Steve Johnson, converting his ninth match point for a 6-4 7-6 (7-4) win over the American.
Canada's Denis Shapovalov overcame American Frances Tiafoe in a thrilling 7-6 (8-6) 3-6 7-5 victory to set up a meeting with Britain's Kyle Edmund.
Japan's Kei Nishikori defeated Russia's Andrey Rublev in straight sets, while 13th seed Pablo Carreno Busta beat Richard Gasquet 6-3 2-6 6-3.