"That crazy first set was so important and the service break at the beginning of the second set too.
"He had been playing some fantastic tennis during the whole tournament, winning against amazing players, and today the first set was super difficult.
"His ball was very difficult to control, very flat, very fast and I am very happy that I survived that."
Nadal is chasing a men's record 21st Grand Slam and coming off a curtailed 2021 season caused by a chronic foot injury followed by a bout of Covid-19 in December.
The story of the match was the extraordinary opening-set tiebreak, where Nadal held seven set points and Frenchman Mannarino four.
It proved an epic in the boiling heat, before Nadal prevailed 16-14 after winning a furious rally.
The 69th-ranked Mannarino appeared to injure his groin or abdomen in chasing down a drop shot during the tiebreak and needed treatment courtside.
The effort of the draining opening set told on Mannarino, who lost his opening service game of the second set and followed up with another in the fifth game.
The outcome was inevitable with Nadal fully in command and getting the job done.
Mannarino was also coming off a 4hr 38min struggle to get past 18th seed Aslan Karatsev in the previous round which finished in the early hours of Saturday.
Mannarino battled on but Nadal gained ascendancy to work his way to victory to remain unbeaten after seven matches this season.