The Australian world number 11's second shot hit some trees before it bounced off a patron's shoulder and into the cup of beer he was holding.
Rather than reach into his beverage the man chugged the rest of the beer and handed the golf ball back to Day, who entered the year's first major as one of the favourites.
Day got a free drop as close to the incident as possible and went on to bogey the par-four first.
The 11-times PGA Tour winner finished the first round at three-over-par 75 after mixing five bogeys with two birdies.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
(Reporting by Frank Pingue; Editing by Ken Ferris)
