RIO DE JANEIRO - Novak Djokovic suffered a second Olympic Games defeat in the space of 24 hours when he and Nenad Zimonjic were knocked out of the men's doubles. The Serb pair lost 6-4, 6-4 to Brazilian fourth seeds Marcelo Melo and Bruno Soares. World number one Djokovic lost his opening round singles to Juan Martin del Potro on Sunday, a defeat which left him in floods of tears. The Games are probably over for Djokovic unless he is nominated for the mixed doubles on Tuesday.