Novak Djokovic overcame a right wrist issue to defeat the Czech Republic's Jiri Lehecka 6-1, 6-7 (3), 6-1 and send Serbia through to the United Cup mixed team quarterfinals. The No. 1-ranked Djokovic and defending Australian Open champion was troubled by his wrist during practice on Tuesday morning, with the injury requiring intense treatment from his physio. Djokovic was cleared to play his singles match and it was just as well after Wimbledon champion Marketa Vondrousova defeated Olga Danilovic 6-1, 3-6, 6-3 to give the Czechs a 1-0 lead.