Fifteen-year-old amateur Lydia Ko posted her second consecutive 4-under 68 on Friday to share the second-round lead of the Canadian Women's Open.

Ko and Chella Choi, who fired an 8-under 64 in round two, completed 36 holes at 8-under 136 and are three strokes ahead at the Vancouver Golf Club.

First-round leader and world No. 1 Yani Tseng reverted back to her uneven form on Friday. She shot a 3-over 75 and fell into a tie for 10th place at 3-under par.

"I didn't play terrible today," said Tseng. "With 3-under, I still have two more days to go and hopefully next two days I can fight back a little bit."

Tseng won three of the first five events on the 2012 LPGA Tour schedule, but after a victory at the KIA Classic, she posted just three top 10s, and they came in the three events following that win.

Tseng missed three cuts in her last five starts prior to this week. She tied for 59th at the LPGA Championship and did marginally better at the U.S. Women's Open, where she shared 50th.

Reigning U.S. Women's Open champion Na Yeon Choi (72), this year's Evian Masters winner Inbee Park (71), Angela Stanford (70) and Moira Dunn (70) share third at 5-under 139.

The whole field is chasing this 15-year-old, who, in January of this year, when she was still 14, became the youngest person ever to win a professional event. She captured the Women's NSW Open on the ALPG Tour.

"It's nerve-racking playing with the pros and actually playing against the pros," said this year's U.S. Women's Amateur champion. "I'm really here for experience."

The goal might have shifted some.

Ko played the front nine even with nine pars. She picked up two pars to start her back nine, then rattled off three consecutive birdies from the 12th to get to 7-under par for the championship.

After pars at 15 and 16, Ko birdied the 17th for the second time in as many rounds. She grabbed the lead at 8-under, parred the last then was joined in front by Choi.

"Today I didn't make any bogeys, and that was really helpful because sometimes a bogey in the middle of your round can make your confidence go a little down," said Ko.

She is certainly a big player already in the game of women's golf, but Ko is still just 15. She admits she misses being a normal teenager.

"I'd love to go out on Saturday night with my friends and watch a movie, but that happens really like once a year or a couple times a year," said Ko. "I sometimes really miss going out with my friends and that's why I actually like school because I can see my best friend there."

Choi, who turns 22 on Saturday, parred five in a row at the start. She birdied five of six around the turn and also had a birdie run on the back nine.

Choi, who is winless in her LPGA Tour career, birdied three in a row from the 13th and parred out her share of the lead.

"I'm very happy," said Choi, who lost a playoff at this year's Manulife Financial LPGA Classic and shared third at the Jamie Farr Owens Toledo Classic. "I worked out really hard last winter for my driving distance, and just focused on my driving distance and my workout. So I'm not tired after third or fourth round."

Suzann Pettersen (69), Jiyai Shin (70) and Vicky Hurst (70) share seventh at 4-under par.

NOTES: Stacy Lewis, who is first on the LPGA Tour money list, shot a 1-under 71 on Friday and is at 1-under for the championship...The 36-hole cut fell at 3-over par and Lexi Thompson, Morgan Pressel, Ai Miyazato and Michelle Wie all missed the cut...Jessica Shepley is the only Canadian to make the weekend...Defending champion Brittany Lincicome shot a 1-over 73 on Friday and is tied for 34th at plus-1.