Rome - Germany's Alexander Zverev became the youngest player in a decade to reach a Masters final when he clinched a gutsy 6-4, 6-7 (5/7), 6-1 win over big-serving American John Isner in Rome on Saturday.
Zverev, 20, will meet either four-time winner Novak Djokovic or Austria's Dominic Thiem in Sunday's final.
Djokovic was the previous youngest Masters finalist, going all the way to the title in Miami as a 19-year-old in 2007.
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 number 14 by reaching the final and would break into the top 10 with the title.
Djokovic, the second seed and the only previous Masters winner still in contention, plays eighth seed Thiem later Saturday after completing his rain-interrupted quarter-final against Juan Martin del Potro to beat the unseeded Argentinian 6-1, 6-4.
Thiem, beaten to the Madrid Masters title by Rafael Nadal last week, avenged his loss by dumping Spain's former seven-time Rome champion out in the quarters on Friday.
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 Ukraine's Elina Svitolina.
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 Romania's Simona Halep.
Svitolina will chase a fourth title of the year after trophy triumphs in Taiwan, Dubai and Istanbul.
Muguruza had needed three sets to beat Venus Williams in the quarter-finals on Friday night.
Sixth-seeded Halep, the champion in Madrid last weekend, reached her first final in Rome after sweeping aside Dutchwoman Kiki Bertens 7-5, 6-1 in her semi-final.
"Sometimes a win over a top player gives you confidence," Halep said after her semi-final.
"Sometimes the work you do before the tournaments gives you confidence that you are ready to go. Sometimes just winning some matches, tough ones, even if in three sets, those are the most important matches."