Muller overcame the American in straight sets, 6-4, 7-6 (5) booking his place in the semifinals at the Queen's Club for the first time in his career, where he will play the highest seed remaining the draw, the fourth seed Marin Cilic, who won the title in 2012 or Donald Young.
Earlier Cilic, who dropped a set for the first time in the week, wrapped up the opening set quickly and had he converted break points in the third or seventh games of the second set he would have maintained his unblemished record into the final.
Conceding the second set, Cilic was keen to come back in the game and win the third and final set to win the.
"In these last two, three months, I've been feeling great on the court and I would say consistency-wise maybe it's the best ever for me".
Regardless of the form of his rivals, Cilic can be confident of making some waves at Wimbledon if he serves with the accuracy and power he showed at Queen's.
The unseeded pair played out a tight, two-set match in the first quarter-finals of the day.
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The 35-year-old avenged that painful loss by beating Dimitrov in the semi-finals on Saturday and now he has his hands on the trophy at last. Cilic has won 10 of his last 12 matches on grass court.
"We had a great battle in the 2014 final so I knew it would be tough", said Lopez, referring to a match that featured three tiebreakers.
The 34-year-old from Luxembourg then rallied to force the decider before Cilic's superior serving saw him through after two hours and 14 minutes, when victory was closed out with a 20th ace.
"It's hard to believe I'm playing my best tennis at 35, but yes I am", he said. "I thought until the end that I wouldn't make it, but I was a little bit lucky and it went my way".
Cilic now looks favourite for the title, especially after the clinical way he dispatched 19-year-old Kozlov with his serve and forehand nearly unplayable at times.
Lopez, for all his elegant grasscourt skills and one of the most devilish serves in tennis, has only made three Wimbledon quarter-finals in 15 visits.