Novak Djokovic’s preparations for his Australian Open title defence were dealt a blow in an injury-affected defeat to Alex de Minaur in the United Cup in Perth.

Djokovic lost 6-4 6-4 to world number 12 De Minaur as Australia took a 1-0 lead in their quarter-final against Serbia in the mixed team event before sealing a 2-0 victory.

Djokovic was below his best and struggled with an on-going problem with his right wrist, twice requiring treatment from the physio.

The world number one needed a medical time-out for the issue in his previous United Cup match against the Czech Republic’s Jiri Lehecka, which he won in three sets on Tuesday.

The Serbian was treated after holding serve to lead 4-3 in the opening set and again after De Minaur had broken to edge 5-4 up.

Despite Djokovic’s fitness concerns, De Minaur produced an impressive performance to seal one of the biggest wins of his career in just over an hour-and-a-half.

De Minaur said in his on-court interview after the match: “It’s extra special. Novak is an unbelievable competitor and what he’s done for the sport is pretty special.

“When you go up against Novak you have to go out and enjoy it and back yourself and no matter what keep fighting until the end. Today was my day and I’m happy that I was able to get the win.”

Ajla Tomljanovic followed De Minaur’s win by beating Serbia’s Natalija Stevanovic 6-1 6-1 to clinch Australia’s semi-final place.