India, Aug. 31 -- Novak Djokovic's latest Grand Slam campaign ended in painful fashion at the US Open on Monday, with the 39-year-old admitting he is uncertain about how much longer he can continue competing at the highest level after a physically exhausting defeat to Mariano Navone of Argentina. The 24-time Grand Slam winner battled through a series of fitness problems before eventually falling 7-6, 5-7, 6-4, 2-6, 6-1 to Navone in the opening round at Arthur Ashe Stadium.

Djokovic's condition deteriorated as the match wore on. Humid conditions appeared to take a significant toll on the Serbian, who was visibly exhausted and struggled to maintain his intensity during the five-set contest. He was seen using an ice bag on his head during t...