Munoz shot a seven-under-par 65 at the Tanah Merah Country Club, putting her two strokes ahead of Swede Karin Sjodin, Americans Lizette Salas and Stacy Lewis, Thai Pornanong Phatlum and Korean Sun Young Yoo.
One shot further back is Paula Creamer, who suffered whiplash injuries in a road accident on her way to the airport in Thailand last Sunday, world number one Yani Tseng, Danielle Kang and Chella Choi.
Tseng shot into contention with a 25-foot putt for an eagle on the 18th hole.
Munoz, who won her first LPGA event last year, was pleased with her approach play at the Sentosa GC.
"I hit a lot of really good second shots to give myself the opportunities - and I managed to convert a lot of them," she said.
"I saw hitting the ball in play as the most important thing but there were times when I took risks. I could have found the bunkers but I felt I needed to be hitting in shorter irons to some of the holes."
Creamer's caddie Colin Cann said she played within herself after struggling for consistency on the range.
"She was hitting one shot straight and the next 20 yards wide of target," he said.
"Paula went with the fade which she found easier to hit and she accepted that she wasn't going to knock the ball as far as she usually does."
Sjodin and world number two Na Yeon Choi, tied for 11th on three-under, gave credit to their caddies.
"My caddie managed to figure out the wind when I was clueless and we hit the hybrid and a few three woods to all the right spots," said Sjodin.
"I think everyone is going to be in those same areas, so it was a matter of hitting greens and getting some putts to fall."
Na Yeon Choi said: "I have to trust when my caddie gives me the yardage and I have to trust myself.
"I like a difficult lie, a difficult shot and a difficult course. That way, I think I learn many things."
Japan's Ai Miyazato pulled out of the event as a result of injuries sustained in the five-car crash in Bangkok which involved Norway's Suzann Pettersen as well as Creamer.
Pettersen is tied for 44th on one-over while American defending champion Angela Stanford is four-over. Her compatriot Michelle Wie is one-under.