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COQUITLAM, British Columbia (AP) — Teen star Lydia Ko shot her second straight 4-under 68 on Friday for a share of the lead with Chella Choi in the Canadian Women's Open.

Ko, the 15-year-old South Korean-born New Zealander who won the U.S. Women's Amateur two weeks ago, birdied four of the last seven holes in a bogey-free round to match Choi at 8 under. In January, Ko won the New South Wales Open in Australia at 14 to become the youngest player to win a professional tour event.

"I didn't make any bogeys, and that was really helpful," Ko said. "My goal was 4 under today, and I shot 4 under on the back nine, which is good once again, and I did that yesterday as well." She's trying to become the youngest winner in LPGA Tour history and the first amateur champion since JoAnne Carner in the 1969 Burdine's Invitational.

"I'm just here for the experience," Ko said. "But the professionals, on the other hand, it's about how much money they're going to get by each placing." The 22-year-old Choi, from South Korea, followed her opening 72 with a 64 on The Vancouver Golf Club course. She rebounded from a sore shoulder that hampered her Thursday.

"I'm very happy," she said. "I don't know how I made the putts." Club member Brian Alexander is serving as Ko caddie — the first time he has ever caddied. "Two weeks ago at the U.S. Amateur, my mom caddied, and that is kind of a different feeling, because she's your mom and you have to listen to her," Ko said. "It was really comfortable having my mom there, but it's also really relieving and comfortable to have someone that knows the course off their hat, really. He's been here for, I think 10 years, so he knows where not to go and where to go. There were quite a few tricky greens."

Alexander, a real estate developer, said he gave Ko tips on the course during practice rounds earlier in the week. However, she has been calling the shots in the tournament. U.S. Women's Open champion Na Yeon Choi, Angela Stanford, Moira Dunn and Inbee Park were tied for third at 5 under. No Yeon Choi had a 72, Stanford and Dunn shot 70, and Park 71.

"(When I was 15) I was a car hop at Sonic," Stanford joked. Suzann Pettersen, the 2009 winner at Priddis Greens in Calgary, Alberta, was 4 under after a 69. "It feels like you're being beaten by a kid," Pettersen said about Ko. "I know she's good. The problem is, she's too young to understand where she's at."

Jiyai Shin and Vicky Hurst also were 4 under. They shot 70. Top-ranked Yani Tseng, the first-round leader after a 66, had a 75 to drop nine shots back. "I still have two days to go and, hopefully, next two days, I can fight back a little bit," Tseng said.

Brittany Lincicome, the winner last year at Hillsdale in Mirabel, Quebec, was 1 over after a 73. Michelle Wie, the 2010 champion at St. Charles in Winnipeg, Manitoba, missed the cut with rounds of 74 and 76.