Alexander Zverev entered the biggest title defense of his career with some question marks surrounding the state of his game—his only tournament win of 2019 was an ATP 250.

Zverev laid some of the doubts to rest on Monday, when, in his ATP Finals opener, he defeated Rafael Nadal, 6-2, 6-4, thumping 29 winners to Nadal's 13.

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Nadal's usual attempts to push his opponent around the court proved ineffective in the face of Zverev. Just look at this 105-m.p.h. forehand Zverev hit while on the run and in the doubles alley, already up a set and a break.

The straight-set win makes Zverev and Stefanos Tsitsipas the two undefeated players in their group; Tsitsipas beat Daniil Medvedev, 7-6 (5), 6-4, on Monday. Zverev and Tsitsipas are set to meet on Wednesday, in a rematch of their China Open semifinal, which Tsitsipas won, 7-6 (6), 6-4.

One of the storylines of 2019 has been Medvedev and Tsitsipas surpassing Zverev in the rankings. The Andre Agassi Group offers Zverev a chance to reclaim his role as the next big thing.