Novak Djokovic of Serbia celebrates after match point against Alexander Zverev of Germany (not pictured) on day twelve of the 2021 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center, September 10, 2021. World number one Novak Djokovic is a match away from one of the highest achievements across sports, with a calendar Grand Slam on the line after he overcame German Alexander Zverev 4-6 6-2 6-4 4-6 6-2 in the U.S. Open semi-final Friday. Mandatory Credit: Robert Deutsch-USA TODAY Sports

Novak Djokovic of Serbia hits a forehand against Alexander Zverev of Germany (not pictured) on day twelve of the 2021 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center, September 10, 2021. Mandatory Credit: Robert Deutsch-USA TODAY Sports

Novak Djokovic of Serbia hits a forehand against Alexander Zverev of Germany (not pictured) on day twelve of the 2021 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center, September 10, 2021. Mandatory Credit: Robert Deutsch-USA TODAY Sports

Novak Djokovic of Serbia hits a forehand against Alexander Zverev of Germany (not pictured) on day twelve of the 2021 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center, September 10, 2021. Mandatory Credit: Robert Deutsch-USA TODAY Sports

Novak Djokovic of Serbia hits a forehand against Alexander Zverev of Germany (not pictured) on day twelve of the 2021 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center, September 10, 2021. Mandatory Credit: Danielle Parhizkaran-USA TODAY Sports

Novak Djokovic of Serbia reacts after winning a break point against Alexander Zverev of Germany (not pictured) in the fifth set on day twelve of the 2021 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center, September 10, 2021. Mandatory Credit: Robert Deutsch-USA TODAY Sports