SAN FRANCISCO, 27 Apr 2015 (AFP) - Ko, the New Zealand sensation who turned 18 on Friday, earned her first victory as a professional at this event last year.
With the win she notched her seventh LPGA Tour title and fifth since the start of the 2014 season.
She fired a two-under par 70 in the final round to finish 72 holes tied with Pressel at eight-under par 280.
Pressel signed for a fourth round 72. She had birdie putts to win both in regulation and at the first playoff hole, but instead remains in search of her third LPGA Tour title and a first since 2008.
Brooke Henderson, the 17-year-old Canadian who led after the second and third rounds, closed with a two-over par 74 to finish third on seven-under 281.
South Korea's Kwak Min-Seo's final-round 74 was good enough for fourth place on six-under 282, and China's Feng Shanshan was alone in fifth after a 72 for 283.