ATP Finals 2019 Day 4 preview: Nadal takes on Medvedev in US Open final rematch, defending champion Zverev takes on Tsitsipas

Alexander Zverev kick-started his title defence at the 2019 ATP Finals with a facile straight-sets win over two-time finalist Rafael Nadal. It was the young German's first win over the World No. 1 in six attempts.

In the other match of the group, Stefanos Tsitsipas bested Daniil Medvedev in a clash of two tournament debutants.

On the fourth day of the season-ending tournament, Zverev returns to action against Tsitsipas while Nadal takes on Medvedev - the latter being a rematch of this year's US Open final.

If Nadal beats Medvedev and Zverev beats Tsitsipas, Zverev would become the first player from the group to qualify for the semifinals. On the other hand, if Medvedev beats Nadal and Zverev loses to Tsitsipas, the Greek player would qualify for the last four.

Let us have a preview of the two matches on the second day of round-robin action in this group.

Rafael Nadal (x1) vs Daniil Medvedev (x4)

Rafael Nadal (left) and Daniil Medvedev

In his ninth appearance at the season-ending tournament, Nadal fell to defending champion Zverev in straight sets for his first defeat in six matches against the young German.

It marked the second time in as many appearances (also 2017) that the Spaniard dropped his opening match at the ATP Finals.

In his second round-robin match at the 2019 ATP Finals, Nadal takes on another young gun in a rematch of the pair's Coupe Rogers and US Open finals - both of which went the way of the World No. 1.

Nadal, who is 5-3 in second round-robin matches at the tournament, has an unbeaten 2-0 record over Medvedev, now a two-time Masters 1000 winner. It is a potential must-win match for both players, as another defeat for either player would seriously jeopardize their chances of qualifying for the semifinals.

The Spaniard needs to win the tournament or hope that Djokovic loses before the semis to ensure that he finishes as the year-end No. 1 for the fifth time in his career (level with Jimmy Connors, Roger Federer and Novak Djokovic).

Stefanos Tsitsipas (x6) vs Alexander Zverev (x7)

Sascha Zverev (left) and Stefanos Tsitsipas

Both Zverev and Tsitsipas started their ATP Finals campaigns with straight-set wins.

On the second day of round-robin action in the group, Zverev and Tsitsipas lock horns with each other. Depending on the outcome of the earlier match in the day between Nadal and Medvedev, the winner of Zverev and Tsitsipas could become the first player from the group to qualify for the semifinals.

If Nadal beats Medvedev and Zverev beats Tsitsipas, the defending champion would qualify for the last four. On the other hand, a Medvedev win over Nadal coupled with Tsitsipas beating Zverev would make the 21-year-old Greek the first semifinalist from the group.

In four previous career meetings between the pair, Zverev has a 1-3 record against Tsitisipas - which includes wins for the Greek in the pair's two meetings this season (Madrid quarterfinals, Beijing semifinals).