Stefanos Tsitsipas shocked six-time champion Roger Federer 6-3, 6-4 to reach the final of the ATP Finals in his tournament debut on Saturday.
The Greek sixth seed, 17 years younger than his 38-year-old opponent, beat the Swiss at the Australian Open but had fallen to him twice since then.
Federer was unrecognisable from the player who dominated Novak Djokovic in his final round-robin match, struggling on serve and hitting a total of 26 unforced errors compared with just five against the Serbian.
But Tsitsipas belied his years with a performance full of confidence and grit, saving 11 out of 12 break points during the match.
“I’m so proud of myself today, a great performance and once again the people were great,” he said.
“I really enjoyed myself on the court and sometimes in matches like these you wonder how you recover from difficulties and break point down.
“It is a mental struggle and I’m proud how many I saved today, I was trying not to give an easy time to Roger, he was playing well.”