The 34-year-old Muller rolled back the years to upset second seed Zverev in Den Bosch -- eventually prevailing 7-6(5), 6-2.
France's Muller, seeded fourth, said afterwards: "It was definitely one of my best matches this season. I played very well from the beginning. Everyone knows Sascha is a tough player and has already won some big tournaments, so I'm very happy to get through.
"It's true that I'm one of the older guys on tour, but I'm having my best time this year. I've been injury-free the past few years and able to play a full schedule, so that has helped me a lot."
Big-serving Karlovic, 38, took a little longer to oust Cilic 7-6(4), 5-7, 7-6(2) and said ahead of his 18th ATP Tour singles final: "It was a great match. I was struggling this year, so it's nice to make another final. Hopefully the final against Gilles is a good match also."