Dominic Thiem beat Argentina's Diego Schwartzman 6-3, 7-6 (7/3).
ATP Finals: Stefanos Tsitsipas is the youngest champion since Lleyton Hewitt in 2001 and the fourth first-time champion at the season-ending event in as many years.
Dominic Thiem saw off the big-serving German 7-5, 6-3 after Greece's Stefanos Tsitsipas, making his ATP Finals debut, had earlier beaten Roger Federer.
Dominic Thiem produced a scintillating display of attacking tennis to beat Novak Djokovic and qualify for the last four at the ATP Finals.
Roger Federer was beaten 5-7, 5-7 on the opening day after Novak Djokovic had swatted aside debutant Matteo Berrettini in the earlier Group Bjorn Borg contest.
The "Big Three" remain the men to beat at the season-ending ATP Finals in London.
Novak Djokovic saw off Britain's Kyle Edmund to reach quarterfinals while Grigor Dimitrov knocked out fifth seed Dominic Thiem in Paris Masters.
Dominic Thiem had to dig deep to get past Milos Raonic 7-6 (7/5), 5-7, 6-4 as the Canadian powered down 30 aces in a thunderous serving display.
Dominic Thiem will face Diego Schwartzman in the final after the Argentine fifth seed beat Gael Monfils 6-3, 6-2 in his semi-final.
China Open: Naomi Osaka said that she felt she had been "counted out" after hitting poor form following her Australian Open triumph in January.
Rafael Nadal needed barely two hours to see John Millman off 6-3, 6-2, 6-2 at Arthur Ashe Stadium.
Rafael Nadal, who won his fourth title in Canada a year ago, needed almost two hours on Friday to overcome the Italian veteran who shocked him on clay at Monte Carlo in April.
Top-seeded holder Rafael Nadal suffered through a two-hour rain interruption before dismissing Britain's Daniel Evans 7-6 (8/6), 6-4.
Roger Federer took time to get going and was helped by Lloyd Harris suffering a calf problem midway through the match.
The victory on Sunday moved Rafael Nadal on to 18 Grand Slam titles, just two behind Roger Federer on the all-time list.
Dominic Thiem will take on Rafael Nadal in French Open 2019 final.
The semi-final will resume at midday (1000 GMT) on Saturday, organisers said
Top seed Novak Djokovic won 7-5, 6-2, 6-2 and will face Dominic Thiem on Friday in what will be his 35th Grand Slam semifinal.