Choi shot a 5-under 67 in the third round, while overnight leader Lewis had a 69.
Paula Creamer, who injured her shoulder in a car accident after a tournament in Thailand last weekend, also had a 69 to move into third place at 12-under 204.
Four golfers were in a group three strokes behind Creamer — American Danielle Kang (70), Spaniard Azahara Munoz (72), South Korean Sun Young Yoo (72) and Thai 17-year-old Ariya Jutanugarn (72).
Lewis and Choi were in the last group on the course at Sentosa Golf Club and had just teed off on the 18th hole when a thunderstorm rolled in, causing play to be suspended for nearly 2 1/2 hours.
Lewis was up a stroke when they left the course, but when they returned, Choi hit a 10-foot birdie putt to pull even with the American heading into Sunday.
"I think that frees me up for tomorrow," Choi, the reigning U.S. Open champion, said of sinking the birdie putt. "I finished strong and that putt, I hit pretty solid."
The two golfers traded turns in the lead throughout the day, with neither able to pull away for good.
Lewis, the 2012 LPGA Player of the Year, bogeyed holes No. 1 and 3 to fall two strokes behind Choi at the start of the round.
The American regained her composure, however, and reeled off three straight birdies — including one with a 25-foot putt on No. 8 that barely caught the edge of the hole and dropped in — to pull even with the South Korean at the turn.
Choi moved atop the leaderboard again with a birdie on the 14th hole, but she bogeyed the next hole after missing a long par putt to give the lead right back.
Lewis maintained the one-shot advantage going into the 18th hole. And then the rain came.
"It was very frustrating," the 28-year-old Lewis said. "It is what it is and luckily we got finished today.
"It's going to be tight tomorrow and we are going to have to make some putts," she added.
Creamer, who was in the next-to-last group, said she felt lucky she barely made it off before the thunder and lightning started.
"Sometimes it has its perks being second to last," she said. "Dinner will taste a lot better, especially not having to grind it out on the last hole."
Creamer, the 2010 U.S. Open champion, had low expectations coming into the tournament after injuring her shoulder in a five-car accident on the way to the Bangkok airport on Sunday night.
The pain was so bad Thursday morning, she said, she wasn't sure if she'd be able to play. She persevered, however, and now sits two shots off the lead going into the final round.
"I'm just so pleased to be out here, and let alone to be in contention on Sunday," she said. "That's just beyond my imagination of what I thought even teeing it up on Thursday."
Also in contention going into the final round is Ariya, the Thai teenager who has posted some impressive results since turning professional in December.
She tied for second at the Australian Ladies Masters in February and nearly captured her first professional title a couple weeks later when she took a two-stroke lead into the final hole at the LPGA Honda Thailand event only to completely fall apart. She wound up with a triple bogey and lost the trophy to Inbee Park.
Ariya hasn't let the disappointing finish affect her this week, however. She made six birdies in a blemish-free second round on Friday, and followed that up with a steady 72 on Saturday.
Ariya's sister, 18-year-old Moriya Jutanugarn, is also in the field in Singapore. She shot a 71 and is in a tie for 19th place with top-ranked Yani Tseng of Taiwan and Spaniard Beatriz Recari at 4-under 212.
Tseng followed up her second-round 73 with a 71 on Saturday. The five-time major winner hasn't captured a tournament in nearly a year and could soon be in danger of losing her No. 1 ranking to Choi, currently in second.
Choi can't overtake Tseng with a victory this week, but she is steadily inching closer — a fact she's trying to keep out of her mind.
"I think that if I think about that tomorrow, then I don't think I can have good results," she said. "So I will try to just think about one shot at a time."