Tseng, fourth in seedings that were based upon last year's LPGA final money list, won six holes on birdies to eliminate Paraguay's Julieta Granada 5 and 3.
Choi ousted Canada's Alena Sharp 3 and 1, taking the 16th hole with a par and the 17th with a birdie to reach the final eight.
In later matches to decide semi-final berths, Tseng will face Norway's fifth-seeded Suzann Pettersen and Choi will play Sweden's Sophie Gustafson, the lowest remining seed in the field at 42nd.
A birdie on the par-five 18th gave Pettersen a 1-up triumph over American Stacy Lewis while Gustafson edged US ninth seed Michelle Wie 1 up.
Gustafson fell 1-down three times on the front nine but answered each time to pull level and took her first lead with an eagle at the par-five ninth.
Wie squared the match with a birdie to open the back nine but the Swede regained the edge when Wie took a bogey at the par-three 12th.
Both followed with four pars, bogeys at the par-four 17th and birdies at 18 to see Wie drop out one round earlier than she did in 2010.
Japanese sixth seed Ai Miyazato and US third seed Cristie Kerr booked an afternoon quarter-final matchup with solid third-round morning triumphs over South Korean rivals.
Miyazato ousted 11th seed Inbee Park 2 and 1 while Kerr dispatched Kyeong Bae 4 and 2.
American Angela Stanford, last year's Match-Play runner-up and this year's 18th seed, ousted South Korean Meena Lee 5 and 4. Stanford next faces 10th seed Paula Creamer, who eliminated American compatriot Brittany Lang 3 and 2.
- AFP
Tags: sport, golf, united-states First posted May 22, 2011 07:22:00