Defending champion Novak Djokovic silenced concerns about his left shoulder as the Serbian breezed past American Denis Kudla 6-3, 6-4, 6-2 to reach the last 16 at the US Open. Djokovic's title defence looked to be in jeopardy in his previous match when he received treatment on his shoulder several times, an issue which he said had impacted his serve and backhand. But he looked to be playing pain free against Kudla, the 16-times Grand Slam champion winning 81% of his first-serve points and turning aside all seven break points he faced against the unseeded American. Kudla, who due to a rain delay earlier this week was playing his second match in as many days, brought incredible energy into the match and raced around the court to make a number of shots that surprised Djokovic but was simply overmatched. Up next for Djokovic will be former champion Stan Wawrinka, who beat Italian Paolo Lorenzi earlier on Friday.