Novak Djokovic walked into the semi-finals of the Madrid Masters on Friday when ninth-seeded Marin Cilic withdrew before their match with stomach problems, officials said.
Cilic tweeted his regrets.
"I am sorry to announce that I must withdraw from today's match. I have had a terrible night dealing with a case of food poisoning," he wrote.
"I am extremely disappointed to have my time in Madrid end in this way. Thank you for the support."