Novak Djokovic has suffered a shock exit in the first round at the Olympic Games in Rio de Janeiro.

The world number one was stunned in straight sets by Juan Martin del Potro in an epic battle lasting almost two and a half hours, as the Argentine wrapped up a 7-6(4) 7-6(2) victory on centre court at the Olympic Tennis Centre.

For Del Potro, this marked the second time in a row that he had ended Djokovic's Olympic dreams, having previously beaten him to bronze in London four years ago.

Although singles gold continues to evade Djokovic, he remains alive in the doubles competition with partner Nenad Zimonjic and will play for a place in the quarter-finals on Tuesday.

Djokovic's early exit from the singles competition leaves second seed and defending champion Andy Murray as the new favourite for gold in Rio.