Justin Thomas flirted with a 59 before settling for a one-shot lead at the halfway stage of the CIMB Classic in Malaysia.
Thomas carded 11 birdies in his second round at the Kuala Lumpur Golf and Country Club, with frustrating pars on his last three holes meaning he signed for a 61 and a 15-under-par total.
The 22-year-old picked up his shots in bunches and was five-under for his first five holes on Friday, birdieing the first, fourth and fifth holes and making an eagle at the third.
After three pars, Thomas made birdies at his next four holes, and two more shots were picked up at the 14th and 15th, the American 11-under for the day and seemingly on his way to a round of 60 or better.
But his momentum stalled and he also failed to take advantage of the par-five closing hole as he had to settle for a 61 and a one-shot lead over Brendan Steele.
Scoring conditions continued to be easy in Friday's second round, and Steele made the most of it with the second best round of the day - a nine-under 63, matched by left-hander Brian Harman.
Steele had seven birdies and an eagle, while Harman made 10 birdies after opening his round with a dropped shot at the first. Harman is four shots back of Thomas on 11-under in seventh place alongside the in-form Kevin Na (66).
Third place is held jointly on 13-under by Hideki Matsuyama, who carded a 66, Spencer Levin, who moved up the leaderboard with a 64 that included six birdies on the front-nine, and first-round leader Scott Piercy who made it round in 69.
The European challenge is led by Alex Cejka and Sergio Garcia on seven-under, but it was a poor day for Ireland's Padraig Harrington, who is 11 strokes back on four-under after a round of 72, and Englishman Luke Donald.
Donald is one of only 10 players to reach halfway over-par - he carded four bogeys in a round of 73 after opening up with a 74 on Thursday.