Alexander Zverev claimed the biggest win of his career on Sunday in London, beating Novak Djokovic in two sets to win the 2018 ATP World Tour Finals. The set scores were 6-4, 6-3.

The underdog claimed a break late in the first set and carried his momentum into the second, serving his way to the end-of-season title. Djokovic, who had been in sensational form throughout the tournament, never got to the level he has displayed in the past few weeks.

Djokovic came out firing in the rallies but found a serious challenger in Zverev, who was game from the base line and did plenty of damage with his serve.

The two had plenty of excellent back-and-forths during the opening set, keeping things close for those in attendance:

Djokovic seemed to have the edge in the longer points, but Zverev's defence was on point and the German did a great job of avoiding errors. He did go to his challenges early, wasting a few on shots that were clearly out, but overall the 21-year-old made few mistakes.

In the ninth game of the set, Djokovic made two crucial errors that handed the youngster a break chance. The German took full advantage, claiming it immediately and serving out the set for a shock lead in the following game.

Per TennisTV, Zverev became the first player to claim a game on the Djoker's serve this tournament:

And he didn't stop there. Zverev carried his momentum straight into the second set, with a rattled Djokovic coughing up another break.

The Serb needed to halt the run badly and tied things up in the next game, courtesy of an error and two double-faults from Zverev. Tennis writer George Bellshaw thought it could be a turning point:

Zverev refused to let the bad serve game get to him, however, using a poor drop shot to earn himself another break point and converting once again.

Live Tennis did not like the look of Djokovic:

The 31-year-old was unable to continue the run of break of serves, with Zverev holding for a 3-1 lead and a golden opportunity to win.

Zverev kept serving well and soon found himself within a single game of the upset, taking care of business in the ninth game.

It's easily the biggest win of the youngster's career, who will hope this ATP World Tour Finals title will set him up for even greater things in the Grand Slams in 2019.

Djokovic had a phenomenal end to the season but seemed visibly tired in Sunday's final, perhaps not surprising given the many matches he has played of late.