Zverev, 20, will meet either four-time winner Novak Djokovic or
Djokovic was the previous youngest Masters finalist, going all the way to the title in
Zverev and Djokovic have never played, but Thiem leads their head-to-head 4-1, including all three meetings on clay.
Zverev will climb to a new career high in the rankings at No 14 by reaching the final and would break into the top 10 with the title.
“I don’t look at these kinds of stats, who is the youngest player to have a Masters 1000 title or things like this. I try to play match by match and try to improve,” said Zverev, who fired 12 aces and 41 winners past the 32-year-old Isner.
Djokovic, the second seed and the only previous Masters winner still in contention, plays eighth seed Thiem later on Saturday after completing his rain-interrupted quarter-final against Juan Martin del Potro to beat the unseeded Argentine 6-1 6-4.
French Open champion Djokovic took his record over the giant Del Potro to 14-4.
Thiem, beaten to the Madrid Masters title by Rafael Nadal last week, avenged his loss by dumping
Second-ranked Djokovic had taken a one-set lead over Del Potro, who held a 2-1 lead in the second set before heavy rain and lightning halted play on Friday.
In the WTA event, French Open champion Garbine Muguruza was forced to retire due to a neck injury from her semi-final against
The Spanish third seed was 1-4 down after just 22 minutes in the first set when she quit, handing the eighth-seeded Svitolina a place in Sunday’s final against
Svitolina will chase a fourth title of the year after trophy triumphs in
Muguruza had needed three sets to beat Venus Williams in the quarter-finals on Friday night.