Dominic Thiem set up an ATP Finals final against Stefanos Tsitsipas after defeating defending champion Alexander Zverev 7-5, 6-3.
Thiem and Tsitsipas will go head to head to win the biggest title of their respective careers in only the third final of the event since 2002 not to feature Roger Federer, Rafael Nadal or Novak Djokovic.
The first set was a tight affair and in the early exchanges, the opportunities fell the way of Zverev, who missed break point chances in the third and fifth games of the match.
Thiem withstood the pressure and after going 6-5 up, he crafted his first two break point chances and converted the second to take the set.
The sixth game of the second set proved to be the decisive one. Zverev squandered a 40-15 lead and was dragged into a lengthy battle, twice saving break point but failing to do so a third time.
Zverev then missed two chances to hit back immediately and it proved to be costly as the Austrian continued his superb run in London to reach the final.
This will be the fourth straight year that the ATP Finals has seen a first-time winner crowned.
"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," Thiem said after his win, which came after a loss to Matteo Berrettini on Thursday where he was hampered by illness.
" 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."
"Sascha was serving 140/145mph most of the time so in the first set break I hit some good returns and in the second set he had some second serves so I used that chance.
"It was key because you don't get many chances to break him.
"I'm feeling much better, luckily. I had some days off, of course I played match against Matteo which wasn't great for my body but I could handle it. I feel much better and I hope it will be better tomorrow.
"I will give everything because it is the final and the last match before holidays."