Novak Djokovic suffered a frustrating fitness saga as a wrist problem led to the Serb's upset 6-4, 6-4 loss to Australia's Alex de Minaur in the quarterfinals of the United Cup in Perth on Wednesday. Djokovic, who received multiple massages on court on his troublesome right wrist -- hurt in training 24 hours earlier -- fell victim to an inspired de Minaur, who led his side into the weekend semi-finals in Sydney. Ajla Tomljanovic then sealed the 2-0 win for the team with her 6-1, 6-1 defeat of Serb 184th-ranked substitute Natalija Stevanovic. Stevanovic had replaced an exhausted Olga Danilovic, who featured in previous ties here alongside Djokovic. World number one Djokovi...Keep on reading: Djokovic wrist injury dooms Serbia to Australia defeat