Denis Shapovalov blew hot and cold as he was twice forced to come from behind to beat Kwon Soon-woo 6-7(5) 6-4 6-4 6-2 and secure a place in the third round of the US Open for a fourth straight year on Wednesday.
The 12th seed was in all sorts of trouble against the unheralded South Korean at the Louis Armstrong Stadium, losing the first set in a tiebreak and going 4-2 down in the third after levelling up the contest.
The 21-year-old Canadian broke back for 4-3 and called a medical timeout to have some treatment on his hip area before storming to victory by winning 10 of the last 12 games.
"Definitely an extremely tough match today," Shapovalov, who made 57 unforced errors, said on court.
"I was up in the tiebreak in the first set, didn't go my way. And to bounce back in the second was really tough. So I'm really happy to get the win today."
Kwon belied his lowly ranking of 73rd in the world with some fine tennis in the first half of the contest, making up for what he lacked in power with great movement and some fine shots.
Shapovalov, though, contributed to the digging of the hole he found himself in, blowing a 5-2 lead in the first set tiebreak with a string of errors and giving up his serve in the third despite a couple of good chances to put the game away.
When his serve was working, however, he always looked like having too much for his 22-year-old opponent and so it proved as he sealed the contest when Kwon netted a return after three hours and 42 minutes on court.
Shapovalov, who went out to Kevin Anderson and Gael Monfils in five-set thrillers in the third round on his last two visits to Flushing Meadows, will next face American Taylor Fritz.