With temperatures at the Billie Jean King National Tennis Center once again nudging towards 100 degrees Fahrenheit (38 C), Wawrinka and Humbert battled for nearly three hours and 30 minutes in the shadeless oven that is the Grandstand Stadium court.
Wawrinka, who won the U.S. Open in 2016 but was unable to defend his title last year after undergoing knee surgery, had his fitness put to the test by the 20-year-old Humbert playing in his first Tour level event.
"My level is there. I'm playing really good tennis," said Wawrinka. "I'm 33-years-old, first time after big surgery that I'm pushing myself that much, but in general, I think I did enough hard work to know and have confidence in my fitness.
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"I think there is good chance that I'm playing better the next round than today, so I'm positive with that."
Humbert let the three-times grand slam-winning Swiss know he was in for a long and sweaty afternoon after the first set went to a tiebreak and he levelled the match by taking the second.
Wawrinka, however, lived up to reputation as a battler and grimly hung on to take the next two sets.
"It is a battle to play a player that never played before, a young player," said the Swiss wildcard. "He's been playing really well this summer, doesn't give you a lot of rhythm.
"It was a great battle. Was for sure tough fitness-wise but I think I handled it pretty well.
Wawrinka, who beat eighth seed Grigor Dimitrov in the opening round, now faces 25th seed Milos Raonic after the big-hitting Canadian blasted past Frenchman Gilles Simon 6-3 6-4 6-4.
(Editing by Ed Osmond/Greg Stutchbury)
