Milos Raonic (Can), 25
World ranking: 6
Best at US Open: Fourth round (2012, 2013, 2014)
He nearly made the final of this year's Australian Open, before losing to Andy Murray in five sets. He defeated Roger Federer at Wimbledon, before getting beaten again by Murray in three sets.
So, in terms of Grand Slam form, Raonic is playing his best tennis.
The Canadian should definitely progress further than his best so far in New York - which surprisingly has only been the last 16.
He has the big serve and the game. But fitness could be an issue.
Marin Cilic (Cro), 27
World ranking: 9
Best at US Open: Winner, 2014
He has hit top form just before the US Open by beating Andy Murray in the Cincinnati final. Earlier, he took Roger Federer to five sets at Wimbledon.
The fast, lower-bouncing surface at Flushing Meadows is made for him as he showed two years ago by thumping Roger Federer and Kei Nishikori on his way to winning his only Grand Slam title.
The Croatian has the big serve, powerful forehand and the footwork needed to win another Major.
Kei Nishikori (Jpn), 26
World ranking: 7
Best at US Open: Final, 2014
Nishikori made history at the 2014 US Open when he became the first Japanese player to finish as runner-up. But in his opening match last year, he was ousted in five sets by France's Benoit Paire.
His confidence has been boosted by winning the bronze at the Rio de Janeiro Olympics, where he lost to Andy Murray in the semi-finals before beating Rafael Nadal in the third-place match.
He is consistent and dynamic and has aggressive footwork. He also manages to find difficult angles.
But the question is whether he has enough firepower to win the US Open.