Against an opponent still on the comeback trail after surgery on his right shoulder, Tsitsipas jumped to a 4-1 lead in the opening set but saw the world number 152 Coric turn the tables for a 7-6(0) 6-2 victory.
Asked if he knew what made the difference after his brisk start, Tsitsipas said: "I'm not in any condition to answer this question, because I'm still looking for the answer myself.
"I don't have a clear answer for that."
Tsitsipas, the 2021 French Open runner-up, has a tour-leading 19 Masters 1000 wins this season, winning titles in Monte Carlo and Mallorca, and reaching the finals in Rotterdam and Rome.
He came into the contest against the Croatian Coric after defeating US Open champion and world number one Daniil Medvedev in the semi-finals of the ATP Masters event, the last big tune-up tournament before the hardcourt major starts from August 29.
"I guess I was too relaxed," Tsitsipas said of the match against Coric. "It cost me.
"He was obviously the opposite of what I was, very much involved. Sometimes it makes you not be that much present in the moment when you're too relaxed. It takes away that momentum that you are trying to capture.
"I think he was serving well. He was making me move a lot. But, yeah, I could have used my chances in the first set. I don't know why I didn't. I rushed a little bit. He had a few good returns, and it cost me because that first set could have been very life-changing."
