LONDON — Despite beating Novak Djokovic in straight sets to win the ATP Finals at only 21, Alexander Zverev knows keeping pace with the Serb isn't going to be easy.

Zverev claimed the biggest title of his career with a 6-4, 6-3 upset on Sunday, becoming the youngest champion of the season-ending event since Djokovic claimed the first of his five titles a decade ago — also at age 21.

"Oh my God," said Zverev, who also became the first German winner since 1995. "I've won one (year-end title). He's won five. He's won, I don't know what, 148 titles more than me. Let's not go there for now. I hope I can do great ... but just chill out a little bit." Top-ranked Djokovic was attempting to tie Roger Federer's record of six titles but followed the same path as the Swiss great, who lost to Zverev in the semifinals at the O2 Arena.