The Korean-born Kiwi was entertained by fans singing 'Happy Birthday' to her at the start and finish of her second round and was even presented with a birthday cake after leaving the final green.
"Really cool to share that birthday with a lot of people out here," Ko admitted after her round.
"I always say, presents are a good thing.
The 18-year-old left Lake Merced to celebrate the occasion over a Korean barbecue dinner with family and friends and admitted to loving the iPhone 6 Plus her mother gave her as a birthday present, but was left disappointed with the way in which she played on the day.
"I really didn't get anything going," Ko said from the clubhouse after her round.
"When that happens, it's really hard. I would say the par-fives, you kind of need to take advantage of them. I didn't position myself well in any of them, and that just makes it tough. Kind of feel like you're already losing shots there."
"Putts weren't really rolling in," she lamented after a round which featured only one birdie and a solitary bogey.