A golf fan had to a get himself a new drink at a PGA Tour event on Thursday ... and it's all 'cause one of the pros hit his approach shot right into his cup.

The crazy moment happened just minutes ago out at The American Express tournament in La Quinta, Calif. ... when Adam Schenk was attempting to hit one of his first-round shots close to the green on the 16th hole.

The 31-year-old, however, pushed his ball to the right ... and when it came crashing down from the sky, it improbably landed in a spectator's beverage!!

The guy holding the cup was stunned ... but in perhaps even more shocking news, once Schenk fished the ball out of the dude's refreshment -- he birdied the hole!!

No word yet if Schenk replaced the fan's drink -- or if he gave him some cash to go grab another one.

But, seems it was all pretty good luck regardless -- 'cause he finished the day with a solid 2-under par score ... and the confidence that he can make it in any hole going forward.