Cilic broke to love in the 10th game of the match to take the first set and kept his nose in front in the second, only to falter badly in the tiebreak which Lopez dominated.
Dimitrov saw his backhand disappear down the stretch, and he was almost up to the task of surviving without it as he saved six break points in a nine and a half minute fourth game of the set to hold for two-all.
Lopez regrouped in the decider, breaking Dimitrov in the sixth and eighth game to book a ticket for the final.
With the top three seeds having been eliminated, the fourth seed, Marin Cilic is arguably now the favourite to win this tournament.
The big-serving duo put on a spectacular show for the crowd in London, with Cilic eventually proving too strong as he ran out a 6-3 5-7 6-4 victor.
Lopez erased the match point on his serve at 4-5 in the deciding set and scored the lone break of the contest in the following game before advancing in two hours and 27 minutes.
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A big serve gave Cilic match point but Lopez saved it with a reaching volley and then released an ace for his own championship point. In the first round, Muller came through in a tight match against the Georgian player, Nikoloz Basilishvili, in 3 sets.
"In the important moments I'm able to bring out some great shots", the Luxembourg player said. "I served extremely good today".
"I'm very happy to be here in the final because it's one of my most special tournaments", Lopez said.
Cilic serves so well and I like the way he backs up his serve now. And also against Gilles today I was mixing it up really good and just playing really well, really smart in some critical points.
Two-time Wimbledon champion Petra Kvitova beat Lucie Safarova in the Aegon Classic semifinals on Saturday in Birmingham, England, to reach the first final since her playing hand was injured in a knife attack at her home. "I have been waiting for so long.to be here holding this trophy", he told BBC Sport.
Lopez, yet another male player in their 30s who appears to be getting better with age, is the oldest victor of the prestigious Queen's Club championship in the professional era.