Boris Becker believes it is hard to pick a favourite for the US Open given the amount of injuries affecting the players at the top of the men's game.
Novak Djokovic, Stan Wawrinka and Kei Nishikori have already pulled out of the final grand slam of 2017 due to injury, and doubts remain over Andy Murray's fitness.
Former world number one Murray has not played since hobbling out of Wimbledon after his quarter-final loss to Sam Querrey, while SW19 champion Roger Federer missed the Rogers Cup after tweaking his back.
Milos Raonic and Marin Cilic are also doubtful for Flushing Meadows, leaving the men's draw wide open, something that Becker believes is exciting.
"In the men's competition I don't want to name any favourite to win the US Open," he told a media conference after being appointed head of German men's tennis.
"The defending champion Wawrinka is in the hospital and has knee surgery, last year's finalist Djokovic has a hurt elbow, so they are out.
"Federer plays a magnificent year until now. On paper he might be the top-favourite. The new number one right now is [Rafael] Nadal, so it is not really clear. At Wimbledon I would have immediately said Federer, and in Paris, Nadal.
"Alexander Zverev has just won two tournaments. The masters in Montreal he won the final against Federer.
"What is Nick Kyrgios going to do? He is also one of the young and wild guys that has what it takes to win a grand slam tournament.
"It is very open and this can make the tournament really exciting."