Defending champion Novak Djokovic admitted he had played “his worst tennis ever” as he slumped to his earliest exit at the US Open in 18 years on Friday, sparking more questions over his long term future in the sport. 

The 37-year-old Djokovic, a four-time champion in New York, lost 4-6, 4-6, 6-2, 4-6 in the third round to 28th-ranked Alexei Popyrin of Australia, leaving him frustratingly short once again of winning a record 25th Grand Slam title.