Many stars on tour know the ins and outs of winning matches, but do they know their history of the sport?

Gael Monfils, Grigor Dimitrov, Frances Tiafoe, Marin Cilic, and Karen Khachanov all sat down to have their tennis IQ tested. The task was simple; name as many ATP world No. 1's as you can. Thankfully, they didn't have to go in order, but it still wasn't an easy task. The ATP created the ranking system in August of 1973, and since then 26 men have held the prestigious title of world No. 1.

Fun fact: The latest new name to be included on the list was Andy Murray in November 2016. And yes, all of the stars named Murray during their respective games.

Newer tennis fans may only be able to name the Big Three. Many of the men started off with them as well. Then things got more complicated. In the end, Monfils was the only person to guess all 26 correctly with Tiafoe coming in last place, remembering only 16 names.