Dominic Thiem reached his maiden ATP Finals final after beating Alexander Zverev on Saturday night.
The Austrian world No 5 came through a grueling tough battle at the O2 Arena to knock the defending champion.
The pair were strong on serve throughout the opening exchanges but then Zverev bust first.
After putting to bed his double fault demons this week, the ghouls returned in a frightening moment of madness.
Thiem earned a vital break point while 6-5 up in the first set but Zverev delivered a double fault to throw the set away.
The German then did similar six games into the second set while allowed his opponent to push for a break on deuce.
And at the third attempt Thiem took the lead and held his nerve on serve to reach the final.
He will play Tsitsipas at 6pm on Sunday after the 21-year-old Greek star beat Roger Federer earlier in the day.
Meanwhile, Thiem says he is looking forward to playing against Tsitsipas in the final.
“This is completely a dream come true for me,” Thiem said.
“It is one of the best tournaments all year and one of the most prestigious.
“I think we are both very offensive players. He is very attractive to watch so I am looking forward to playing him.
“I am very lucky and looking forward to playing in front of this amazing atmosphere once again.
“I am feeling much better luckily. I had some days off since I played Matteo [Berrettini]. I am feeling much better and I will give everything.
“It is the last match of the season and after that it is the holidays.”
