Sinner won 7-6 (7-4) 6-1 against Spain’s two-time major winner on Tuesday, and now leads their head-to-head 4-3.
The 22-year-old Italian said his performance was at the “very top” of those he had produced in big matches.
Medvedev beat Alexander Zverev 6-4 6-3 in the other semi-final.
Russia’s world number three, 27, has won all six of his previous matches against Sinner.
The Italian, who will rise to a career-high of fourth in the world after the tournament, says he plans to “change a couple of things” tactically for Wednesday’s final in Beijing.
He can also take plenty of extra confidence from a superb display against Alcaraz.
Using his powerful groundstrokes to unsettle the 20-year-old Spaniard, Sinner won a first-set tie-break in which he never trailed and then saved five break points on his way to dominating the second set.
“I would say every match against him is very tough. We always show great respect, we both play great,” Sinner said about his exciting rivalry with Alcaraz.
“When we play against each other, we try to stay on our limits.
“I played a little bit better in the important moments. In the second set, it was 6-1 but he had a lot of chances and I just tried to stay focused.
“The first set was a little more of a rollercoaster and I tried to stay there mentally and obviously I’m very happy with the performance.”