Top seed Djokovic appears to have the toughest task when he faces Juan Martin del Potro in his opener on Saturday. The Serb was defeated by the giant Argentine in straight sets in the bronze medal match four years ago. Djokovic has an 11-3 record against Del Potro, although the two have not met for three years. Del Potro, the 2009 US Open champion, comes into Rio with a world ranking of 145 having been as high as four in 2011 before his career was almost ended by four wrist operations. Djokovic said he never had any intention of pulling out of Rio where five of the world’s top 10 — Roger Federer, Stan Wawrinka, Milos Raonic, Tomas Berdych and Dominic Thiem — will be missing.