Coric, 59th in the world, won 6-3 6-3 as the Scot's desperate pursuit of consistency went on.
His first serve, which has been hindered by an elbow injury, was not to blame against Coric, who played aggressively and fluently and took full advantage of Murray's error-strewn performance. "That's even more when I said (to myself): "I'm going to make even more balls, I'm going to make him play" and just see what's going to happen".
"Novak is someone that I don t have good record (against)", said the Japanese.
And defending champion Novak Djokovic also eased into the quarter-finals with a 6-4, 7-5 win over Feliciano Lopez.
But after failing to capitalise the Scot looked distracted as he remonstrated with his coaching staff as the second set went with serve until a bad eighth game from Murray gifted Coric the break once more.
"Making mistakes very early in a lot of the rallies, I wasn't building any points so I didn't help myself find a way into the match to start playing better and that was disappointing because you're not always going to play your best tennis but you can still find ways to make it hard for your opponent and I didn't do that at all".
Djokovic faces a much tougher test on Friday against world number eight Kei Nishikori, who earlier swept aside David Ferrer 6-4, 6-3.
Thomas lobbies for calls with Celtics-Wizards series tied
His suspension will likely mean more run for the Washington starters, and possibly more playing time for reserve Bojan Bogdanovic. Now, the Boston Celtics have a lot to worry about as they head back to the TD Garden. "Damn sure", John Wall said.
"I got to stay really positive and I got to play great tennis to beat him".
After suffering his own struggles of late, Djokovic was encouraged by his display in seeing off Lopez and a partisan crowd to reach the last eight.
Dimitrov looked in fine fettle as he took moved a set and a break up against Theim, but the 23-year-old Austrian showed tremendous resilience to advance to a quarter-final against Coric.
For the second time in his young career, the Croat has faced Andy Murray in a tournament as a lucky loser, this time at the ATP Masters 1000 event in Madrid.
Women's defending champ Simona Halep has crushed American Coco Vandeweghe 6-1, 6-1 to reach the semis.
Svetlana Kuznetsova ended Eugenie Bouchard's impressive run to the quarters in convincing fashion 6-4, 6-0.
France's Kristina Mladenovic also made it through to the final four.
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