Tseng, the world's No. 1, held the first-round lead after she fired a 6-under-par 66 in a successful return to the golf course where she won her first pro tournament in 2007. Choi was just one shot back, with a five-under 67, after leading for much of the day.
Lydia Ko, the 15-year-old South Korean-born New Zealander who won the U.S. Women's Amateur two weeks ago, was two strokes back at 68 along with South Korea's Inbee Park.
Tseng, who won a CN Canadian Women's Tour event in her first visit to the Vancouver Golf Club, birdied five of her final six holes. She confirmed her notice of two days earlier that she is "back" after struggling the past two months.
"I wish I didn't finish today," said Tseng. "I wish I could keep going — keep playing. But 18 holes are finished. (Friday) is a new day, but I'll try to keep that momentum going and try to do the best I can."
The Taiwanese star won three of the first five tournaments this year, but has struggled since then. Starting her round on the 10th tee, she made a number of long putts on soft greens that resulted from a morning rain that turned into afternoon sunshine.
Noting she also drove the ball straight, Tseng said the return to the links where she first won enabled her to rediscover her lost confidence.
"If I miss a couple shots, I don't worry about it, not like I did before, so I feel that this is the way I used to play," said Tseng.
Meanwhile, Choi excelled with help from a Vancouverite who was thousands of miles away. The 24-year-old South Korean praised her English tutor Greg Morrison, a Vancouver native who is now based in South Korea, for giving her considerable insight on his hometown.
"This is my first trip in Vancouver, but I feel very comfortable, because I heard a lot of things (about the city) from my English tutor," said Choi.
Morrison travelled with Choi last season on the LPGA Tour but chose to remain in his adopted home this year in order to spend more time with his four-year-old daughter. Choi, a Seoul native, said she decided to take English lessons to be able to connect better with fans. The move has also had a positive effect on her golf game.
"Two years ago, I couldn't talk much with the media, with the fans, but right now, I can talk with them," she said. "So I feel I'm really comfortable."
Choi also felt "good vibes" from members of Vancouver's Korean community who came out to cheer her on. She also enjoyed a traditional Korean meal the night before.
"I go to a Korean restaurant (for) every meal if I can, and (Wednesday) night, I found a great restaurant," she said. "(The food) was just like my mom cooked."
Lorie Kane of Charlottetown was the top Canadian after shooting an even-par 72. Fourteen-year-old Brooke Henderson of Smiths Falls, Ont., the youngest golfer to ever play in the tournament, shot a 77.
No Canadian has won the tournament since Jocelyne Bourassa prevailed in a tiebreaker in 1973.
Choi is looking for her second victory of the season after posting a four-shot win at the U.S. Women's Open.
The five-year LPGA veteran got off to a strong start with a birdie on the par-4 second hole and also recorded three straight birdies on Nos. 5 through 7.
She extended her lead quickly after making the turn, making birdies on 11 and 13. But she shot back-to-back bogeys on the next two holes before rallying for a birdie on 17.
"I was losing my focus at that moment," said Choi. "That's why I got two bogeys. But after two bogeys, I realized I had to focus my game and I (had) to go back and get back on track. When I had (the) birdie on 17, that gave me a great feeling. I feel like I have a lot of confidence about my game."
Meanwhile, Kane earned top Canadian honours a day after saying she was not ready to relinquish her role as the nation's torchbearer for women's golf.
"I played a good round for most of the round," she said. " I played some pretty good golf. A couple of things kind of got away on me."
She retained her torchbearer status by rolling in a birdie putt from about eight feet on the final hole.
As a result, Jessica Shipley of Oakville, Ont., was the second-best Canadian with a 1-over 73. Shipley was one shot better than American star Michelle Wie.
After three-putting on the final hole, Wie cursed and threw her putter. She found herself in a tie for 100th after finishing second in this tournament last year in Mirabel, Que., and winning it in 2010 in Winnipeg.
"I tapped in and I missed and, on that last one, I didn't feel too good ... But other than that I had a really solid round," said Wie.
Her show of frustration generated considerable buzz. Kane called for people to be fair with Wie, predicting the former teen phenom will fare better on the tour now that she has graduated from Stanford with a communications degree and can devote herself to a regular golf routine.
"The LPGA needs Michelle to be playing well, because she moves the needle," said Kane. "If Tiger (Woods) moves the needle on the men's side, Michelle moves it on this side."
Notes: Choi also excelled with help from world No. 1 Yani Tseng's former caddy. Jason Hamilton carried Tseng's bag when she won three of the first five tournaments this year, but he was fired in July after she struggled. Choi said Hamilton will work for her full-time.