Novak Djokovic is hailing Alexei Popyrin as a legitimate US Open title contender after the audacious Australian rocked the Serbian superstar's grand slam dreams in New York.
Djokovic rued "some of the worst tennis I have ever played" after coughing up 14 double-faults in his 6-4 6-4 2-6 6-4 third-round loss to Popyrin on Friday night (Saturday AEST).
"Honestly, (my) serving (was) by far the worst ever," he said.