Lewis had a final round 8-under-par 64 Sunday and won the RR Donnelley LPGA Founders Cup by three strokes. It was her seventh LPGA championship -- five of which have come in the last 11 months.
That gives her a rankings point average of 9.75, which moves her from third past Tseng and Choi Na-yeon to No. 1. She is the seventh player to hold the No. 1 position in the women's rankings.
Tseng had been No. 1 from Feb. 14, 2011, through Sunday, a total of 109 weeks that is second all-time to Lorena Ochoa's record 158 weeks atop the rankings.
Tseng's average falls to 9.13 and leaves her ranked No. 2 while Choi drops from second to third at 8.47.
Inbee Park is fourth at 7.98 followed No. 5 Feng Shanshan (6.63) and Jiyai Shin, whose average of 6.52 is sixth best.
Ai Miyazato, in front much of the week at the Founders Cup until the charge by Lewis, goes from ninth to No. 7 this week with an average of 6.44. Suzann Pettersen (6.14) is No. 8 and Ryu So-yeon (6.04) is ninth. Both are down one place this week.
Mika Miyazato, with an average of 5.02, completes the Top 10.