Just seconds after the 36-year-old won a grueling, 3-hour-and-49-minute match over Carlos Alcaraz in Ohio ... he ditched his top to commemorate the occasion.

At first, Djokovic dropped to the ground in disbelief over his thrilling win ... and then, after sharing an embrace with Alcaraz, he decided to tear the polo straight off his body.

Djokovic ripped the piece from his neck down -- revealing a shredded bod -- and everyone in attendance loved it.

Of course, the celly was much deserved ... Djokovic needed every ounce of his talent to beat Alcaraz in a final that featured 90-degree heat and multiple lengthy rallies.

Afterward, Djokovic told media members the evening was "one of the exciting matches I've ever played in any tournament."

"It felt like a Grand Slam," he said.

Djokovic became the oldest man to ever win the event ... locking up a victory over Alcaraz for the first time since the 20-year-old beat him at Wimbledon last month.

"You never give up, do you?" Djokovic told Alcaraz after the match. "I love that about you."

He went on to say he hopes to compete against the young star at the U.S. Open later this month.