ATP Tour Finals champion Alexander Zverev started the defence of his title in style on Monday, when he beat top seed Rafael Nadal 6-2, 6-4. 

The German won four games in succession to clinch the first set before an early break in the second set him up for a memorable victory over the 19-time Grand Slam champion.

Earlier in the day, Stefanos Tsitsipas got his competition off to a strong start, as he edged two tight sets against Daniil Medvedev. 

Here are the results from Monday's play and a recap of the action, as well as the schedule for Tuesday's matches.

Monday Results

(6) Stefanos Tsitsipas bt. (4) Daniil Medvedev: 7-6 (5), 6-4

(7) Alexander Zverev bt. (1) Rafael Nadal: 6-2, 6-4

Tuesday Schedule

(3) Roger Federer vs. (8) Matteo Berrettini

(2) Novak Djokovic vs. (5) Dominic Thiem

Monday Recap

Those in attendance at the O2 Arena in London would have anticipated a tight match between Nadal and Zverev, or perhaps for the former to ease to a comfortable win. However, it was the seventh seed who settled the better.

After sharing the first four games, the German moved through the gears and capitalised on some uncharacteristically sloppy play from his opponent. After grabbing a break to put daylight to Nadal, it was clear Zverev was full of confidence.

Per Tennis TV, the defending champion was able to absorb and respond to anything Nadal threw at him:

After clinching the set, Zverev kept his foot down at the start of the second, taking another early break and continuing his impressive sequence, per Jose Morgado of Record:

At this point, Nadal had been knocked off his game and he was failing to establish any kind of rhythm against his opponent. Zverev continued to play solid tennis on his own serve and was constantly getting his own returns back into play.

It meant there was never any feeling that Nadal would claw his way into the match and Zverev, who has struggled closing out contests against elite players previously, would have been delighted with the ruthlessness with which he ended the tie.

Per Enrico Maria Riva, Nadal wasn't able to conjure a single break point in the match:

Earlier in the day, Tsitsipas showed little signs of nerves on his first-ever ATP Tour Finals appearance, as he was able to beat Medvedev in a tight encounter.

Both players served brilliantly in the opening set, with a tiebreak following. In it, it was the Greek who played the better tennis and he was able to nudge ahead.

In the second set, Tsitsipas was able to preserve his high standards and produced this excellent volley on his way to levelling the stanza at 2-2:

The Greek had to wait until the ninth game of the set to notch the only break of the match though, as he moved 5-4 in front. Given Medvedev had struggled to cope with his opponent's serve throughout the match, it was no shock to see Tsitsipas serve it out.

Afterward, Tsitsipas played down talk of animosity between the two players; that's after he labelled Medvedev's style "boring" at the Shanghai Masters last month:

On Tuesday, Roger Federer will be under pressure to earn a result after his shock loss on Sunday but will be a big favourite against Matteo Berrettini.

Dominic Thiem, who beat Federer in that encounter, will continue his challenging start to the tournament against Novak Djokovic.