Choi, a two-time winner each of the last two years, is winless in 17 starts this season. She was second last week behind Tseng in South Korea and has five top-10 finishes in the last six events.
'I played well the last two months,' Choi said. 'I finished in contention almost every week, so I feel great about my game. Every year in the beginning of the season play so so and then getting better, better. 'So, think my style is gradually getting better at the end of the year.'
Lang is coming off a 13th-place tie last week in South Korea.
'I think I really simplified things,' Lang said. 'I really just tried to get out of my own way and have more fun and just go back to kind of playing like you do when you're a kid. I've had more energy, I've had more fun, and I'm playing better. I'm just happy that I came to that realization.'
The players took advantage of soft conditions in the event increased from 54 to 72 holes this year.
'I guess they've got a lot more rain, which makes the course easier, because last year it was tricky with the firm greens, so I'm thankful for that,' Lang said. 'That's probably the only change. It's in great shape. Everything's the same, just the greens are softer.'