John McEnroe explains one thing that can stop Novak Djokovic's US Open Calendar Slam bid

McEnroe still backed the 34-year-old to win the US Open, despite being unable to win his first gold medal.

He added: “I felt before Wimbledon started that he was going to win the Grand Slam. I felt like he was going to lose at the Olympics. The two-out-of-three format, just excessive travel, no fans, et cetera, I think contributed.”

Despite showing strong support for the three-time former champion at the US Open, McEnroe named the one thing he believed could affect Djokovic’s chances.

 “I think the heat is the big thing,” he said. “To me it’s going to be him against the field. I pick him right now.”