The Serbian made several uncharacteristic mistakes but was still able to emerge from a hard-fought contest with a 3-6, 7-6, 6-3 victory.

Djokovic, who admitted before the tournament began that he was struggling to find the same motivation before he was forced to take an extended break away from the game through injury following his French Open triumph earlier this year, will take on Spain's Roberto Bautista Agut in the last four.

The 15th seed produced a fine display to cause an upset as he overcame Frenchman Jo-Wilfried Tsonga, seeded ninth, in straight sets.

Bautista Agut proved far too strong on the day as Tsonga toiled as he wrapped up a confident 6-3, 6-4 triumph in this ATP Masters 1000 tournament. 

"I wasn’t striking the ball very well from the beginning of the match, to be honest," Djokovic said. 

"I was quite flat with my feet, so he got me where he wanted to get me, and to stand still at the baseline. 

"He was fighting, he was focused. 

"He gave his best, and he was a few points from winning."

Britain's Andy Murray, the reigning Olympic and Wimbledon champion, remains on course for a showdown with Djokovic in the final after he swept aside Belgium's David Goffin 6-2, 6-2.

Murray, the orld number two, did endure a few nervy moments, particularly in the second set where he had to save two break points with the scores locked at two games all.

The Briton's reward in the semi-finals will be a clash with Frenchman Gilles Simon, who dropped the opener in his clash with American Jack Sock before completing a 4-6, 6-4, 7-6 win.

"It was a tough match," said Murray. 

"I struck the ball really well and created a lot of chances on my return game."