The world No 3 enjoyed a three-stroke advantage overnight following her superb eight-birdie 64 on Friday, and she began where she had left off on day three as she birdied the opening hole to get to 15 under.
But her form dramatically deserted her as she bogeyed four of the next five holes to gift the outright lead to playing-partner Amy Yang, who followed a birdie at the first with 10 consecutive pars.
Yang then broke the sequence with a bogey at the 12th, and Lewis got back on track with a birdie at 13 before pitching in for an eagle two at the 15th.
The American suddenly found herself four clear of Yang after the South Korean dropped her third shot in five holes at the 16th, but she hit back with birdies at 17 and 18 while Lewis scrapped her way to an ugly six at the long last.
“The eagle at 15 was much the only good thing that came out of today,” Lewis said afterwards. “I was a bit all over the place. I played six really bad holes to start the day but then got things going.”
The resulting 73 left Lewis just a single shot clear of Yang (71), while Sandra Gal was one of only a handful of players to break 70 in hot and humid conditions at the Siam Country Club as she moved into outright third place.
The German birdied three of the last five holes to return a four-under 68 and get within two shots of the leader, while South Korean pair Mirim Lee and Jenny Shin both fired 70 to close on 10 under par.
“I was kind of in the zone and just really playing one shot at a time,” said Gal. “It was good to finish strong in this humidity and this hot weather where it’s really tough to stay focused and fresh especially during the end of the round.”
Former world No 1 Yani Tseng is a further two shots off the pace after a 70, while Solheim Cup stars Suzann Pettersen (68), Caroline Masson (74) and Azahara Munoz (71) will start the final round seven behind Lewis.