Djokovic, who recently triumphed in winning the Olympic gold medal in Paris, appeared fatigued and lacked his usual sharpness against the spirited Australian opponent.
Novak Djokovic experienced a significant upset at the hands of Alexei Popyrin in a 6-4, 6-4, 2-6, 6-4 defeat during the third round of the US Open on Friday, effectively halting the Serbian's pursuit of a 25th Grand Slam title.
Djokovic, who recently triumphed in winning the Olympic gold medal in Paris, appeared fatigued and lacked his usual sharpness against the spirited Australian opponent, who secured the most notable victory of his career.
"I spent a lot of energy winning the gold, and I did arrive to New York just not feeling fresh mentally and physically," Djokovic told reporters.
"But because it's US Open, I gave it a shot and I tried my best. I didn't have any physical issues. I just felt out of gas and you could see that with the way I played."
Alexei Popyrin, who suffered defeats at the hands of Novak Djokovic in both the Australian Open and Wimbledon earlier this year, celebrated a significant victory by defeating the defending champion and advancing to the round of 16 in a major tournament for the first time. Popyrin raised his arms in triumph and flashed a wide smile as he achieved this milestone.
In the match, Djokovic had five opportunities to break Popyrin's serve in the first set but was unable to capitalize on any of them. On the other hand, Popyrin seized his first chance to break Djokovic's serve, taking a 5-4 lead and ultimately winning the first set.
Popyrin continued his strong performance in the second set, breaking Djokovic's serve for a 3-2 lead and ultimately securing the set with a well-executed serve and volley. Djokovic managed to stage a comeback in the third set by breaking Popyrin's serve, but Popyrin quickly regained control in the fourth set with a powerful forehand winner.
Despite Djokovic's uncharacteristic performance, which included 14 double faults, Popyrin's aggressive play and 50 winners secured him the victory. This loss marks the first time since 2017 that Djokovic will finish a season without a Grand Slam title.
Popyrin's upset victory on Arthur Ashe Stadium followed another surprising result, with third seed Carlos Alcaraz falling to unseeded player Botic van de Zandschulp in straight sets. Popyrin will now face American Frances Tiafoe in the fourth round.
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 "Defending champion Novak Djokovic knocked out of US Open in shock defeat to Alexei Popyrin")
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