With Rafael Nadal having bowed out from the ATP World Tour Finals injured, the question is who can stop the Roger Federer juggernaut.
Just four players have beaten Federer this season but one of them, Alexander Zverev, will be on the other side of the net at the O2 on Tuesday night.
The 20-year-old is being talked about as the outstanding player of his generation, and has the knowledge of his Rogers Cup Final win over Federer in August to inspire him.
Zverev has won five tournaments in 2017 including two Masters titles. He said: “I think anyone beating Federer in this group has a good chance of passing (into the semi-finals). But he’s definitely the favourite in all of the matches he plays. I played him a few times this year now.
“All of them were great matches. Hopefully it can be another one.”
It could be the best match of the round-robin stage, and may merely prove a taster before a repeat in Sunday’s final. The men’s game has long been crying out for a player to step up and fill the impending void to be left by the big four, who have between them endured a litany of injuries in the two years.
Zverev is seen as the player to do that, the youngest man in the world’s top 50, and only Federer and Nadal have won more titles in 2017 - but he has never been beyond the fourth round of a Grand Slam.