Fourth seed Cilic battled past Gilles Muller of Luxembourg 6-3 5-7 6-4 following an hour's delay to the original order of play, while Lopez came through 7-5 3-6 6-2 after an interrupted encounter with Grigor Dimitrov in the other half of the draw later on Centre Court.
Lopez's second serve hasn't been as effective, but he's been broken just once this tournament and has forced his opponent to play a near perforcent match to win.
Cilic, 28, lifted the Queen's trophy in 2012 and finished runner-up to Andy Murray a year later, but must now finish off the doubles semi-final which he was partnering Marcin Matkowski against Jamie Murray and Bruno Soares, which was not finished on Saturday because of rain.
"I think I was playing some of my best tennis during the whole week and hopefully I can keep that running tomorrow".
It was Muller who cracked first as Cilic broke for a 4-2 lead that sealed the first set.
Cilic is guaranteed to be at least No. 6 in the rankings, which is a career high, after dealing Muller a first loss of the grass-court season following the left-hander's title win in Netherlands last week.
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The two are joined on the bench by Tommy Seymour, while Greig Laidlaw is the only Scot to start. We are going to have some quite lengthy debates in terms of finalising that back three.
Cilic, a victor at Queen's in 2012, had coasted through the opening set against Muller, who was broken for the first time in this year's tournament in game six.
"Against Gilles today I was mixing it up really good and just playing really well, really smart in some critical points".
The Aegon Championships 2017 final between Marin Cilic and Feliciano Lopez is set to start at 2:00 pm local time, 6:30 pm IST.
"I can not believe that I have finally won this trophy". "I played him here past year and had a match point". Notably, it was the former, who dumped out Wawrinka with a straight set win over the Swiss star in his tournament opener.
When Cilic's serve is clicking the way it is right now, he's one of the tougher players on tour to beat.
It proved to be a momentary lapse, and Cilic converted his second match point when fired down a second-serve ace.
The second set again saw both players hold serve without too much trouble, sending it into a tie-breaker.
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