As Alexander Zverev and Novak Djokovic battled for a spot in Sunday's US Open final, they gave the fans in Arthur Ashe Stadium a show with a record 53-shot rally. It's the longest rally in a US Open men's singles semifinals ever -- passing the 51 from Andre Agassi and Pete Sampras in 1999 -- Djokovic's longest rally of the year and the longest rally of the tournament. 

The rally came as Zverev was down 15-40, with the German finishing the exchange off via an inside-out forehand winner. Djokovic eventually won the set, 6-4, giving him a one-set advantage. 

Friday's matchup between Zverev and Djokovic determines who plays Daniil Mendelev in Sunday's final. Mendelev punched his ticket to the US Open final with a win over Felix Auger-Aliassime earlier in the day. 

Zverev is searching for his first Grand Slam title while Djokovic is hunting for a record 21st. A Djokovic win in the final would also give him the first calendar-year Grand Slam since 1969. 

These two have brought fireworks to the court before, as they battled in the 2020 Tokyo Olympics semifinals. Zverev upset Djokovic en route to Olympic gold.