The challenge of returning Ivo Karlovic’s serve is among the greatest in tennis, and with the best in the game often struggling to get his notoriously big delivery back in play, one could say it would be a fool’s errand for mere mortals to make the attempt.

Nevertheless, four such fools—albeit some of the hardest-working fools at the US Open—took on the task ahead of the final Grand Slam fortnight of 2021.

How did they fare? Let’s just say the 6’11” giant known as “Dr. Ivo” pitched a perfect game.

The Croat has been among the most dangerous floaters for a generation, bursting onto the scene in 2003 when he dethroned defending Wimbledon champion Lleyton Hewitt in the first round. Ranked as high as world No. 14 in 2008, Karlovic has won 8 ATP titles and made his best Grand Slam result at the All England Club in 2009.

Now 42 years old, Karlovic made it through qualifying to earn his 63rd Grand Slam main draw appearance, and admitted to considering retirement following his first-round exit to No. 5 seed Andrey Rublev—though he may yet change his mind.

One thing's for sure: wherever he plays next, beware his all-but-unbreakable serve.