Red Eléctrica, Spain’s electricity grid operator, ruled out a cyberattack, human error or unusual weather. Experts say it could take weeks to determine the cause.

Play was first suspended soon after 12.30 p.m local time, at an inopportune moment for Britain’s Jacob Fearnley. He had just saved a match point and broken Grigor Dimitrov to keep their match alive, but the players were taken off court with the Brit about to serve down 5-4 in the second set.