The second-round battle lasting for more than two and a half hours was the first match for the 39-year-old with 24 Grand Slam singles titles since he lost to Jannik Sinner in the Wimbledon semi-finals more than a month ago.
Djokovic, who claimed the last of his three Cincinnati titles in 2023, claimed the opening set with little fuss.
He won a brutal 18-minute third game of the second, saving four break points over nine deuces but bending over in distress between points and even dropping to his hands and knees after one particularly gruelling exchange.
Djokovic took a medical timeout after that game, with the physio and then a doctor coming on court to assist with assessments and ice towels.
After Tirante took the second set both players cooled off in the locker room.
Slightly revived, Djokovic set the pace again in the deciding set.
But it all went wrong in the ninth game, when Djokovic saved three break points before getting it wrong on a drop shot to bring up a fourth break chance that his opponent converted.
Tirante clinched the biggest win of his career in the next game as Djokovic again failed to land a drop shot on match point.
The Serb, who must now try to regroup before the US Open starts on August 30, admitted he might have played his last match in Cincy.
“It looks like I won’t be back, unfortunately to play here,” he said, confirming that the sweltering weather got the better of him.
“It was a combination of the heat and humidity, all those things,” he said.
“Everything involved makes it worse.”
Seventh seed Flavio Cobolli, runner-up at the French Open, defeated Miomir Kecmanovic 6-1, 4-6, 6-3.
Martin Landaluce beat Matteo Arnaldi 6-4, 4-6, 6-4 to reach a third-round clash against Tirante.
In the WTA draw, Philippines sensation Alexandra Eala advanced to the third round 6-1, 3-0 as injured Elena-Gabriela Ruse quit their match during a weather delay.
The Romanian had received treatment on her right ankle just before play was halted by a thunderstorm.
During the 90-minute delay, Ruse pulled the plug on her effort, sending ninth seed Eala through.
Eala, the emerging WTA star who captured her first WTA title in Washington this month, will next play Amanda Anisimova after the American ninth seed beat Turk Zeynep Sonmez 6-2, 6-3.
As always, world number 20 Eala was supported by a vocal contingent of compatriots, with Philippines flags displayed in abundance.
The 21-year-old has enjoyed similar support around the world — in her run to the second week of Wimbledon, her breakthrough win in Washington and at this months’ 1000 event in Toronto, where she was beaten in the fourth round by Swiss veteran Belinda Bencic.
Eala and Ruse swapped breaks in the first two games before Eala ran off six straight games to pocket the set and was in command when the storm struck.
“I was playing well and feeling really focused,” Eala said.
“I tried to stay in the zone. A retirement is not the way you want to end a match, I wish her a speedy recovery. Injuries are not fun.”
Roland Garros finalist Maja Chwalinska advanced 6-2, 6-2 over Spain’s Cristina Bucsa, the Pole getting her first win since Paris after suffering an injury at Wimbledon and taking a month away.
Anna Kalinskaya ousted Cincinnati local Caty McNally 6-3, 6-4, while Magdalena Frech defeated Jelena Ostapenko 6-1, 6-3. (AFP)
