Defending champion Lydia Ko, the world's top- ranked amateur, fired a 5-under-par 65 on Thursday to grab a share of the lead after the opening round of the Canadian Women's Open.

Last year, Ko became the youngest winner in LPGA Tour history when she titled at this event as a 15-year-old. Now 16, the New Zealand native finds herself tied atop the leaderboard with Angela Stanford and Cristel Boeljon through 18 holes at Royal Mayfair Golf Club.

"It was a pretty special week last year ... so to kind of have that kind of week again this week would be like a double miracle for myself," admitted Ko.

Paula Creamer and Cristie Kerr carded matching 4-under 66s to share fourth, while world No. 1 Inbee Park, Na Yeon Choi, Karine Icher and 22-year-old Canadian Jennifer Kirby, who played alongside Ko, all shot 3-under 67s.

"I felt very comfortable out there and ended up making a few putts, and good round," said Kirby.

Ko and Kirby went out as part of the morning wave, with 17-year-old European Solheim Cup team member Charley Hull completing the threesome, and the defending champion pointed the way early.

A birdie at the first gave Ko the early momentum, which she sustained with further gains at the third, fifth and seventh to turn at 4-under.

Ko began her inward stretch in similar fashion, notching a birdie at the par-4 10th before moving to 6-under with another birdie at the 14th. The amateur phenom promptly gave that stroke back with a bogey at No. 15, but she parred out to retain a share of the lead at minus-5.

"I did feel a little bit of pressure, not from others but from myself thinking because you're the defending champion people are going to expect more and I'm going to expect more from myself," she explained. "I called my dad a couple days ago, and he said 'Just relax. You can't control everything, and just play the game that you want to play and that you planned.'"

Stanford also went out early, matching Ko's mark with a bogey-free effort which included five birdies over her first 12 holes.

"I hit it really well today, really solid, and I've been hitting it solid," said Stanford. "I think finally the ball found the bottom of the hole a little bit faster. I think if you can keep it in the fairways on this golf course, you'll have chances at birdie."

Boeljon teed off late and made her move even later. She played her first 14 holes at 1-under, but picked up four strokes in a three-hole span from No. 15, highlighted by a hole-in-one at the 126-yard, par-3 16th.

NOTES: When Ko won this event last year she became the first amateur to win on tour since Joanne Carner in 1969 ... Boeljon hit 9-iron at the 16th ... World No. 2 Stacy Lewis withdrew after the first round due to illness ... Only one Canadian has won an LPGA event on home soil (Jocelyne Bourassa, 1973 LaCanadienne Golf Championship).