Shanghai - Roger Federer was given a point penalty as he followed Novak Djokovic out of the Shanghai Masters quarter-finals in a bad-tempered defeat to Alexander Zverev on Friday.
The 22-year-old German won 6-3, 6-7 (7/9), 6-3 as the Swiss legend Federer was penalised after twice hitting the ball out the court in anger.
The sixth-ranked Zverev faces Austria's Dominic Thiem or Matteo Berrettini of Italy in the semi-finals on Saturday.
The 20-time Grand Slam champion Federer saved five match points to force a deciding set, but he then became engaged in a continual war of words with the umpire.
The 38-year-old's trademark cool evaded him as the new generation of men's tennis stars took centre stage in Shanghai.
His point deduction came at 3-0 and 30-15 down in the third set, as he threatened to unravel.
Stefanos Tsitsipas said that he scares the "Big Three" after the young Greek stunned world number one Djokovic in their quarter-final.
The 21-year-old has now defeated Djokovic, Federer and Rafael Nadal this year to burnish his fast-growing reputation.
On Saturday though he faces the red-hot 23-year-old Daniil Medvedev - who he has never beaten.
Tsitsipas recovered from losing the opening three games in just seven minutes to shock the reigning champion Djokovic 3-6, 7-5, 6-3.
Calling it the best comeback of his nascent career, the seventh-ranked Tsitsipas said he had dreamt of beating the "Big Three".
But rather than being intimidated by them, Tsitsipas said: "I honestly feel like they are more threatened than I am and that makes me more relaxed on court.
"I know I just have to hang in there and be able to be more aggressive.
"Once you get aggressive and they see you going for it, I might even say they get scared."
It was a double celebration for Tsitsipas, who also confirmed his place in the prestigious season-ending ATP Finals.
Tsitsipas sealed the deal with a gleeful smash and held both arms aloft. Djokovic met his opponent with a warm embrace at the net.
Djokovic, who had looked imperious up until now, had no complaints.
"He deserved the victory, he was the better player," said the 32-year-old Serbian.
"Second and third set I started off very well, but then I wasn't sharp.
"I lacked that little bit of dynamic movement and acceleration in my shots."
Asked if Tsitsipas has the potential to be number one, Djokovic replied: "He definitely has, yes."
Djokovic says that he is fully recovered from a shoulder injury and he did not drop a set in winning the title in Tokyo last week.
He had carried that ominous form into Shanghai and threatened to blow Tsitsipas away in the first set.
But Tsitsipas found his feet in the second set, then went 3-1 up in the decider when Djokovic planted a forehand wide on the third break point.
Tsitsipas grabbed his place at the ATP Finals after fourth-ranked Medvedev tamed self-styled "wild horse" Fabio Fognini 6-3, 7-6 (7/4).
Fognini was vying for a ticket to the Finals in London.
US Open finalist Medvedev has beaten Tsitsipas in all their four matches, but he said: "They were all really tight. He's improving also every week.
"He's still so young. He has even, let's say, two years of more improving than me, talking about age.
"So let's see who wins first."