The five-time major winner has been No. 1 for more than two years, but South Korean Na Yeon Choi and American Stacy Lewis, who captured the HSBC Women's Champions on Sunday, are rapidly closing the gap.
In the new rankings released Monday, Tseng had 9.53 average world-ranking points, while Choi had 8.85 and Lewis was just behind with 8.84, jumping from fourth to third.
Choi and Lewis have made up serious ground recently.
At this time last year, Tseng topped the rankings with 16.69 average points, with Choi far back at 9.03 in second and Lewis at 6.23 in the eighth spot.
Tseng said Sunday the pressure of being No. 1 for so long had taken a toll on her, contributing to her mid-season slump last year.
She hasn't won a tournament since the Kia Classic last March.
"It's tough and it's very lonely," she said. "No one knows how do you feel.
Everybody wants to be in your shoes, but no one knows how tough is that." "The first year, when I was world No. 1, I feel good," she added.
"But every month, everybody keeps building the expectations on me and that's lots of pressure."
The Taiwanese golfer started to find her form again at the beginning of this season, with two top-3 finishes at her first two tournaments in Australia and Thailand.
But she was clearly annoyed after finishing with a final-round 74 in Singapore on Sunday, flicking her ball in the water as she walked off the 18th green.
She'll take next week off to work on her swing, then return to defend her title at the RR Donnelley LPGA Founders Cup in Phoenix in mid-March.
"World No. 1, I know it's good and people like it, but I want to care about myself more," she said. "If I lose (it), I'll get back one day, too." - AP