Dominic Thiem will face Stefanos Tsitsipas in the ATP Finals final after the Austrian beat defending champion Alexander Zverev.
A break of serve in each set saw Thiem, 26, through 7-5 6-3 in an hour and 34 minutes.
Thiem, twice a French Open runner-up, said: "This is a big, big dream coming true for me, it is one of the biggest and most prestigious tournaments of the whole year and I'm getting the chance to play the final.
"It is unreal to me and to beat the defending champion, a very good friend and unbelievable player - it is an unbelievable achievement. I'm very, very happy."