Justin Thomas hit a costly bogey on the 18th hole as Danny Lee grabbed a share of the lead heading into the final round of the CJ Cup in South Korea.
Graeme McDowell is in a tie for seventh on nine under after a third round 68.
Thomas, the champion at Nine Bridges two years ago, had scored a nine-under par 63 on Friday to sit two shots clear.
After dropping a shot at the par-three second on Saturday, the American soon recovered with a 20-foot birdie and then picked up one more on the ninth to go into the turn at one under.
Another bogey at the short 13th was again quickly recovered, only for Thomas to then knock his approach into the par-five last into the water hazard and then find the rough at the back of the green.
Despite avoiding more than one dropped shot, it proved a costly error which allowed Korean-born New Zealander Lee to make up ground as he sunk a fine 60-feet eagle put.
Thomas signed for a two-under 70 while Lee - who had earlier made a double-bogey six at the 10th - recorded a 68.
Australian Cameron Smith sits in third place after he also went four under for the third round, despite needing a seven at the par-five ninth.
Americans Wyndham Clark and Jordan Spieth are tied in fourth at 11 under, with Byeong Hun An a shot back.
Collin Morikawa and Gary Woodland, last year's runner-up, were both big movers after their respective seven-under rounds.
The Americans are in a tie for seventh alongside England duo Ian Poulter and Tyrell Hatton as well as Northern Ireland's McDowell.
Defending champion Brooks Koepka, meanwhile, had earlier withdrawn because of a knee problem.