Kiwi golf star Lydia Ko has shown a cool temperament to defend her Swinging Skirts Classic LPGA title in San Francisco.

The world No.1 birdied the second play-off hole against American Morgan Pressel to snatch the $US300,000 ($A383,000) prizemoney on Sunday after starting the day four shots off the lead.

It is her seventh career victory in her short but remarkable career – she now has earnings just shy of $US3 million ($A3.84 million) since she turned professional in October, 2013.

Ko, who celebrated her 18th birthday on Friday, had started her final round with two bogeys and had two more on the 12th and 16th holes, but crucially sank a sixth birdie on the 18th to tie with Pressel and force the play-off.

Pressel, 26, who had been seeking her first win since 2008, only managed an even score after missing a three-metre putt on the 18th in her final round. She also failed to sink winning putts in the two play-off holes.

Last year, Ko beat American Stacy Lewis by one shot to win the inaugural Swinging Skirts Classic.

Canadian 17-year-old Brooke Henderson finished in third, one shot off both Ko and Pressel.