Second seed Rafael Nadal lost 6-7(9), 6-4, 3-6 to Borna Coric in his Cincinnati Masters match on Aug. 17.
It was only Coric’s fifth win since returning from shoulder surgery in March.
Nadal then play the first match after withdrew from semifinals Wimbledon suffered a torn abs in July.
The number two seed moves slowly, lacks force and serves poorly.
Nadal made seven double faults all game, his scoring rate from serve two just 56 percent.
The two-hour, 51-minute bout was the test for Nadal ahead of the U.S. Open.
As scheduled, he only played at the Cincinnati Masters and took a week off.
If he wants to play more matches to regain his ball sense, Nadal must attend next week’s ATP 250 Winston-Salem Open.
Coric and Nadal spent 76 minutes in the first set.
Nadal had the only break-point in 12 games, but failed to capitalize.
The two players tussled each point in the tie-break.
Nadal missed the previous two set-points, including a missed volley at 6-5.
Coric then capitalized on his third set-point success to win 11-9.
Nadal played better in the second set, holding on to serve thanks to an 83% on-serve scoring percentage.
The 36-year-old made just six self-defeating errors, half as many as juniors.
Coric didn’t have a break-point in the set, and Nadal successfully capitalized on his only chance in game seven to win 6-4.
Nadal’s serve was less effective in the third set, as he hit only a 42% one-on-court ball rate and a 45% win rate from serve two.