The British number one raced to a 6-1 6-3 win in 75 minutes to set up a clash with defending champion David Ferrer.
Murray, 29, beat the Spaniard when he won the Vienna title on his previous visit to the Austrian capital in 2014.
The Scot could overtake Novak Djokovic at the top of the rankings with titles in Vienna and next week in Paris.
That would require Djokovic to lose before the
final in Paris - and Murray to significantly extend his winning streak, which now stands at 13 matches.
He is trying to win his third consecutive title following victories in Beijing and Shanghai as he tries to capitalise on a chance to top the rankings for the first time.
Murray took his record against Isner to 7-0 with a one-sided victory, breaking the 6ft 10in American's serve four times and playing superbly to come back from 0-40 down midway through the second set.
Isner had treatment for a blister on his right hand early in the second set, and Murray said: "It obviously helped me as from the back of the court he wasn't really generating much power with his forehand.
"I was then able to dictate most points from the back of the court. I think I returned pretty good, though. It was important to put him under a bit of pressure, especially from the beginning of the match."
errer saved a match point and won the last four games to beat Serbia's Viktor Troicki 6-3 3-6 7-5, while Frenchman Jo-Wilfried Tsonga and Croatia's Ivo Karlovic made it through to the other semi-final.
Top seeds Jamie Murray and Bruno Soares suffered a surprise 1-6 6-3 11-9 defeat by Austrian Oliver Marach and France's Fabrice Martin in the doubles semi-final.
SOURCE: BBC SPORTS
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