The South Korean admitted to being "excited" to pull off a "perfect" shot, holing out from 167 yards on the par-four 18th to open with a stellar eight-under-par 62 in the first event of the season-ending FedEx Cup play-offs.
"I was just trying to, like, (see) 10 feet on left side and it was (a) perfect shot, (it) came off," he told reporters.
"Yeah, I was excited," he added.
Spaun, his American co-leader, had eight birdies at TPC Southwind in Memphis as he said it was "hot out here".
"I'm hot, figuratively and literally, I guess. It's hot out here," Spaun told reporters.
"I think I putted really well and I drove it really well. I guess I hit 15 out of 18 greens, so that's a pretty good combo. When you're hitting a lot of greens and putting well and hitting fairways, it's hard to complain about how I played today."
Spaun's compatriot Sahith Theegala was at seven-under after a bogey-free round, followed by a group of four a further shot back - Sepp Straka, K.H. Lee, JT Poston and Tony Finau, who is coming off back-to-back victories on the PGA Tour at the end of July.
Rickie Fowler, Jason Day and Tyrrell Hatton are among those at five under.
Two-time FedEx Cup champion Rory McIlroy carded an even-par 70, along with Scotland's Russell Knox, to be eight strokes off the pace.
This week's field features 125 players, but only the top 70 in the FedEx Cup standings will advance to next week's BMW Championship in Delaware.
Only 30 players will qualify for the Tour Championship at Eastlake in Atlanta, Georgia the following week.