(L-R) Stefan Kozlov, Ulises Blanch, Marcos Giron and Noah Rubin attend a Cavaliers NBA game during the Cleveland Challenger.

Stefan Kozlov, Taylor Fritz and Mackenzie McDonald, three of 15 Americans competing in the Houston main draw, at the players' party on Sunday evening.

Stefan Kozlov lifts his second ATP Challenger Tour trophy, prevailing in Las Vegas.

American Stefan Kozlov and Argentina's Guido Andreozzi battle on Day One of US Open qualifying in the shadow of Arthur Ashe Stadium.

#NextGenATP American Stefan Kozlov meets his young fans in Cincinnati ahead of the tournament.

Despite almost falling halfway through the race, Kozlov beat his countryman for bragging rights at the Bash of the Beach in Newport on Sunday.

NextGen stars Stefan Kozlov and Taylor Fritz joke with each other before their stand-up paddle board race this morning in Newport.

Following his first-round win at the Aegon Championships, #NextGenATP's Stefan Kozlov meets Kevin Spacey.

Introducing the Miami 2017 ATP University graduating class!

[L-R:] Taylor Fritz, Daniil Medvedev, Borna Coric, Karen Khachanov, Reilly Opelka and Stefan Kozlov at the register for tickets launch event in Indian Wells for the Next Gen ATP Finals, to be held in Milan, Italy in November.

[L to R:] Taylor Fritz, Daniil Medvedev, Borna Coric, Karen Khachanov, Reilly Opelka and Stefan Kozlov take part in the tickets launch event in Indian Wells for the Next Gen ATP Finals, to be held in Milan, Italy, in November.

Taylor Fritz, Daniil Medvedev, Borna Coric, Karen Khachanov, Reilly Opelka and Stefan Kozlov attend the #NextGenATP event in Indian Wells.

Stefan Kozlov pulls double Hot Shot duty at the ATP Challenger Tour event in Tempe.

Teens Michael Mmoh (L) and Stefan Kozlov teamed up in Sarasota doubles.