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The Korean-born Kiwi withdrew after just two holes of the 2017 BMW Championship with a lower back injury and was unable to feel his legs the next day.
"I was freaking out and asking my wife, 'Are we going to open up a Korean barbecue restaurant now?'" Lee joked after finishing round one a shot behind leader Koepka.
Defending PGA champion Koepka fired a course record 63 at New York's Bethpage Black to top the leaderboard at seven-under-par.
But 28-year-old Lee fired a 64 in the afternoon wave.
Lee's score is remarkable given he hit just seven of 14 fairways and 10 greens, spending considerable time in Bethpage's brutally thick rough.
But 21 putts gave Lee hope of chasing down Koepka and the former child prodigy says he now has the firepower to remain high on the leaderboard.
"I wasn't hitting it far enough to compete in majors but now I can carry my driver about 295 (yards)," Lee said.
Lee feared for his career in 2017 when he tore a ligament between the L4 and L5 discs in his back.
"When I got up from my bed, I could not move my legs. I've never had that kind of injury before," he said.
Apart from 2015, when Lee earned his only PGA Tour victory, he has had to fight to retain his status on the American circuit.
He has admitted it has not been the career he envisioned after becoming the youngest ever winner of the US Amateur Championship in 2008, beating Tiger Woods' record by six months.
He also won the European Tour's Johnnie Walker Classic as an amateur in 2009.
Now world No.119, Lee says playing among the bottom end of the PGA Tour is not as glamorous as it seems.
"Some of the top 20 guys in the world make it look easy, but it's not always fairytales and unicorns out here," Lee said.
"Outside of 100 in the world, your endorsement money is not great. I mean, even I wouldn't pay (a player) outside 100," he joked.
Lee was two shots off the pace after the first round of the 2015 PGA Championship, eventually won by Australia's Jason Day.
But Texas-based Lee is confident he can finish off the tournament better this time.
"This is the first time I've got to play in a major with this distance," he said.
"I'm actually interested in myself what I can do out there this week."