WATCH: Mackenzie McDonald wins a set serving underhand

The saying goes that every time you attend a professional sporting event, there’s a chance you’re going to see something you’ve never seen before.

That theory was proved true on Friday, when Mackenzie McDonald won a set serving underhand.

Seriously.

Facing Canadian Peter Polansky at the Challenger event in Knoxville, TN, McDonald served normally for the first seven games—taking a 4-3 lead in the first set—before having to take a lengthy medical timeout. When he returned, he served underhand and somehow, someway, managed to force a tiebreaker and win it 10-8.

Avoiding embarrassment, Polansky leveled the match after taking the second set 6-4. But to McDonald's credit, he kept it competitive despite the clear disadvantage of not having a true serve at his disposal.

Polansky took five of the first six games in the deciding third set before the 21-year-old Californian decided to retire.

To watch the full match, click here.

McDonald, ranked 293rd, has never won an ATP tour-level match (0-4). Polansky, 28, is ranked 148th.