VANCOUVER, 24 Aug 2015, (AFP) - The 18-year-old New Zealander and world number two beat Lewis on the first playoff hole to add this year's title to her 2013 win as an amateur in Edmonton and in 2012 when she won at age 15 on the same Vancouver Golf Club course.
"I made some really good up-and-downs on the front nine, I hit a good shot into three, but I couldn't really get anything going," said Ko, who was unable to take home the winner's check the first two times because of her amateur status.
"I wasn't hitting anything close. I was, 'hey, I need to make some birdies.'
"Stacy played awesome, so did the other girls, so I knew I needed to play consistently, and that's what I tried to do."
Ko made a short par putt on the playoff hole to record her third victory of the season and eighth of her LPGA career. She and Lewis both shot 12-under 276 totals.
"It has been such an amazing week," said Ko, of Auckland. "I definitely share this win with everyone. I want to thank everyone for their support."
Lewis, who fired a final-round 67, missed a nine-foot par attempt before Ko made the winning three-foot putt.
"If you would have told me at the beginning of the day that I would be in a playoff I would have been pretty happy," Lewis said.
"I left a few out there. I would have liked a better lie in the rough in the playoff. But other than that, it was pretty good."
Defending champion Ryu So-Yeon shot a final-round 64 and finished in a tie for third with fellow South Korean Kim Sei-Young at 11-under 277.
Alison Lee and Candie Kung, of Taiwan, tied for fifth at 10-under.
England's Charley Hull and China's Feng Shan-shan tied for seventh at nine-under 279.
Canadian teenager Brooke Henderson fired a five-under 67 to finish in a tie for 23rd at four-under 284, eight shots back of the leaders.
The 17-year-old Henderson was playing her first event as an LPGA Tour member after winning the Portland Classic last week.
Leading scores on Sunday after the fourth round of the LPGA Canadian Open (x-won in playoff, USA unless noted, par-72):
276 - Stacy Lewis 68-70-71-67, x-Lydia Ko (NZL) 67-68-69-68
277 - Ryu So-Yeon (KOR) 71-72-70-64, Kim Sei-Young (KOR) 69-71-69-68
278 - Alison Lee 70-70-66-72, Candie Kung (TPE) 69-64-71-70
279 - Charley Hull (ENG) 69-73-67-70, Feng Shan-shan (CHN) 70-72-67-70
280 - Park In-Bee (KOR) 71-69-70-70, Lexi Thompson 72-72-66-70, Karine Icher (FRA) 65-70-72-73
281 - Lee-Anne Pace (RSA) 71-71-72-67, Hur Mi-Jung (KOR) 70-73-69-69
282 - Lee Mi-Hyang (KOR) 72-70-74-66, Kim Hyo-Joo (KOR) 71-68-74-69, Ji Eun-Hee (KOR) 71-69-72-70, Lee Il-Hee (KOR) 71-74-67-70, Ariya Jutanugarn (THA) 75-68-68-71, Jaye Marie Green 73-68-69-72, Lin Xi-yu (CHN) 70-66-73 73