The Serbian world number one, winner in Madrid last week, secured an eighth successive victory over the Spaniard, and his 14th and 15th consecutive sets, but not without an almighty struggle.

After claiming the first 7-5, Djokovic recovered from 4-2 down in the second, saving five set points before winning the decisive tie breaker 7-4.

He remains on course for another final showdown with Andy Murray, loser in the Madrid final last week, after the Briton swept past Belgium's David Goffin 6-1, 7-5.

Murray will now face lucky loser Lucas Pouille after his opponent Juan Monaco of Argentina withdrew injured.

It continued a superb week for world number 52 Pouille, who only entered the tournament after French compatriot Jo-Wilfried Tsonga withdrew.

Djokovic will face either Kei Nishikori of Japan or Austrian Dominic Thiem in his last-four clash.

Spain's Garbiñe Muguruza advanced to her first semi-final of the season in the women's draw with a straight set win over Timea Bacsinszky of Switzerland, 7-5, 6-2.

Romanian giantkiller Irina-Camelia Begu also continued her good form with a 6-2, 7-6 win over Misaki Doi of Japan.