Djokovic had earlier been treated by his trainer during the first set, after having dropped his serve.
After losing his serve again to concede the set 6-3, making 18 unforced errors, he decided not to continue, denying him the chance of reaching an 18th successive ATP Tour final.
The Serb was not immediately available to explain his decision, but Lopez told the media that his opponent had suffered an eye infection.
Lopez now meets Marcos Baghdatis in the semi-finals on Friday for the right to meet Nick Kyrgios or Stan Wawrinka in the final.
(Reporting by Neville Dalton; editing by Toby Davis)
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