On Wednesday, Djokovic saved two set points in the opening set before returning to his usual form and defeated his 20-year-old opponent 7-6 (2), 6-4 to reach the pre-quarterfinals of the ongoing tournament.
"I didn't feel health-wise 100 per cent in the last couple of days, but it is what it is. It's not the first time I didn't play at a really high level. Lots of errors, especially in the first set," the ATP Tour website quoted Djokovic as saying.
"First time against a young player who is talented, very quick. Returns a lot of balls back that usually other guys wouldn't get, he gets it. And he was pumped, and I respect his fighting spirit.
"It was a good match in the end, and I found a way to win, which in (these) circumstances... when you're not feeling your best, is important," he added.
The Serbian will next meet Briton Kyle Edmund who beat 14th seed Diego Schwartzman of Argentina 7-5, 6-3 for a place in the quarter-finals.
Meanwhile, Rafael Nadal, who is guaranteed to usurp Djokovic at the rankings summit next week ahead of the ATP Tour Finals in London, beat home wildcard Mannarino 7-5, 6-4 to set up a clash with three-time Grand Slam title-winner Stan Wawrinka.
( With inputs from IANS )