For only the third time in 16 years the final of the season-ending tournament in London will not feature at least one of the ‘big three’ after 21-year-old Greek Tsitsipas beat Federer 6-3 6-4 in the last four.
Nadal, 33, Djokovic, 32, and 38-year-old Federer have ended the year as the top three in the rankings – just as they did as far back as 2007.
But the performances of Thiem – who beat both Federer and Djokovic this week – and Tsitsipas suggest the gap may finally be closing going into next season.
“Does it feel like it might be the best year yet? Possibly,” said Federer. “Then the only issue is that it seems like me, Novak, and Rafa are healthy, healthier than maybe in previous years, as well.
“So from that standpoint, I think they have proved their point this year by qualifying for the Finals, and also now Dominic showed again at these Finals, and winning five titles, beating all of us top guys.
“Definitely it’s that next step they need, and seems like Dominic is in his absolute prime right now. Yeah, I would think so.
“But then I look at the list of who finished world number, who has been world number one all these years, and it’s just crazy that it’s always one of us.
“But we are not getting any younger. So their chances increase not because we are getting worse but because they are getting better, I believe.”
Federer was unable to reach the heights he scaled while dismantling Djokovic two nights ago.
Despite forcing 12 break points he converted just one, while Tsitsipas broke the 20-time grand slam champion three times.
“I’m proud of myself, how hard I fought, how concentrated I stayed in the break points,” said Tsitsipas. “I didn’t crack under pressure. I was very composed and very mature in my decisions.”
Thiem, the world number five, saw off defending champion Zverev 7-5 6-3.
The Austrian had never made it out of the group stages in three previous appearances in North Greenwich, but he has been the stand-out player all week.
Thiem, twice a French Open runner-up, said: “This is a big, big dream come 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.”