"Gilles was just pushing me to the limits", said Cilic, who won the Queen's title in 2012.
Lopez, Cilic and Muller had all reached the semi-finals without dropping serve, so it was a surprise when Cilic began to read Muller's swinging delivery early on and broke in the fifth game of the day's first semi-final.
Cilic, who also finished as Queen's runner-up in 2013, was pushed hard by Young, but the former US Open champion fired 11 aces to see off the American world number 55 in one hour and 26 minutes.
Croatia's Marin Cilic in action against Spain's Feliciano Lopez during their final match at The Queen's Club tennis tournament in London, Sunday June 25, 2017. Amazingly, Muller took it at the first time of asking with a forehand stretch volley victor, winning his seventh successive match in the process. "The last few years have been really good for me, considering the results and also the way I was playing".
Muller, however, managed to find a slight chink in Cilic's armor when he put the Croatian into a 0-40 hole - the first break points all week against his serve. I was dreaming to play.
"But to be able to win two matches in a row gives me more confidence than winning the tournament seven years ago".
Cilic, ranked seventh in the world, is among those in with an outside chance at Wimbledon, where his aggressive game and demon serve can be exploited to the full.
"(Muller) is going to be the first guy that I'm going to meet, apart from (John) Isner, serving and volleying, and obviously with his lefty serve he's really dominant on his service games on grass. "I was waiting so long so I'm so happy today".
Lopez gained a measure of revenge for his loss to Grigor Dimitrov in the 2014 final by beating the Bulgarian 7-5, 3-6, 6-2 in the second semifinal, which was delayed mid-match by rain. "I think it was good today that I played against Young, a lefty swinging serve".
An agitated Cilic held on for another tie-break and this time it was neck and neck, Lopez whipping away a winning forehand off one foot for a 5-4 lead only for Cilic to pat a passing shot inside the baseline for 5-5.style="text-align: center;"