Novak Djokovic delivered a lesson in Grand Slam tenacity to Learner Tien in the first round of the U.S. Open on Sunday, battling past the American teenager 6-1 7-6(3) 6-2 to launch his latest quest for a record 25th major title.

At the U.S. Open tonight, Novak Djovokic’s path to 25 Grand Slam titles begins on Arthur Ashe against 19-year-old American Learner Tien. Tien, ranked 48 in the world, is one of a rising crop of tennis teenage stars on tour right now. Here’s what to know about him as he competes against Djokovic: Khuong coached him until he was around 10 years old.

Novak Djokovic’s first match at the US Open was a bit of a litmus test for the Serbian legend because he didn’t play a hardcourt warm-up tournament coming into the year’s final Grand Slam. The 24-time Grand Slam champion said he didn’t play the Cincinnati Open because he prefers to focus on the Grand Slams at this stage of his career.