WATCH: Zverev comes of age after growing up on tour

Alexander Zverev became an ATP Finals champion at just 21 years old on Sunday, but he's come a long way to get here—especially considering the time he spent on tour with his older brother, Mischa.

"I was born with a tennis racquet in my hands," he told the ATP at age 14, citing how his big brother and both of his parents were tennis players.

Two years later, at 16, he won the junior Australian Open.

And, in 2016, at 19 yeas old, he defeated Stan Wawrinka in St. Petersburg to win his first ATP title.

The next year, he won his first ATP Masters 1000, by taking out Novak Djokovic in the Rome final.

And he quickly confirmed it wasn't a fluke by winning in Montreal, scoring a victory over Roger Federer.

This season, Zverev captured his third ATP Masters title, in Madrid.

And he closed his year in London with back-to-back wins over Federer and a seemingly unbeatable Djokovic, both in straight sets.

Having accomplished all of these feats, Zverev has to be eying a Grand Slam trophy as the next thing on his checklist.