May 12 (Reuters) - Defending champion Novak Djokovic reached the semi-finals of the Madrid Open without hitting a ball on Friday when his quarter-final opponent Kei Nishikori withdrew minutes before the match was due to start with a wrist injury.
The same injury forced world number eight Nishikori, seeded sixth in Madrid, to miss the Barcelona Open last month. He said he aimed to be at next week's Rome Masters.
"Unfortunately I'm still not 100 percent so I'd prefer not to take a risk," the Japanese player told reporters.
"I'm going to take a couple of days to see how the injury evolves but I hope to be in Rome."
In Saturday's semi-final, Djokovic will play the winner of Friday's match between Belgian David Goffin and four-time winner and home favourite Rafael Nadal.
Also on Friday, Croatian Borna Coric, conqueror of world number one Andy Murray in the third round, plays Dominic Thiem of Austria, with the winner meeting either German Alexander Zverev or Pablo Cuevas of Uruguay in the other semi-final. (Reporting by Richard Martin; editing by John Stonestreet)