London - Alexander Zverev beat Roger Federer 7-5, 7-6 (7/5) on Saturday to reach the title decider at the ATP Finals in a match that ended in confusion after a ball boy's fumble forced a point to be replayed.
The 21-year-old German, leading the charge of the new generation against the old guard, is now just one win away from the biggest title of his career but the Swiss must wait for a tilt at his 100th tournament victory.
The end of the match was overshadowed by an unusual incident, Zverev stopping a rally with Federer leading 4-3 in the second-set tie-break after the ball boy dropped a ball.
Zverev won the replayed point with an ace and nudged into a 5-4 lead before Federer inexplicably netted with a simple forehand volley to hand the German two set points, the second of which he took on his own serve.
Immediately after the match finished there were boos from the pro-Federer crowd at London's O2 Arena and on-court interviewer Annabel Croft told them to be "more respectful" as Zverev had been playing by the rules.
"Obviously the crowd didn't appreciate it, which I was a little bit upset -- not upset, but I was a little bit sad at the end with the booing and reaction of the crowd," he said.
"Maybe they didn't know what actually happened. That was maybe part of the reaction. But, you know, at the end of the day I said sorry to Roger at the net. He said 'Look, you don't even have to apologise'."
Federer described it as a "big call" but said Zverev was not to blame.
"It was obviously a big call. Instead of being in the rally in a decent position, you get aced, yes, it makes a difference," he said. "It could have made a difference. That's all hypothetically speaking now at this point."
"I didn't think I was hitting my spots very well for the majority of the match," he added. "I think it came towards the end, which helped me to make it competitive and close at the very end."
Serve dominated the first set of a high-class match, with neither player able to make serious inroads, Zverev sending down howitzers around the 140 miles-per-hour (225 kilometres per hour) mark.
The Swiss six-time champion eventually faltered in the 12th game of the opening set, watching helplessly as Zverev zipped a magnificent passing shot past him down the line on his way to breaking to love.
The second seed regrouped and drew first blood in the second set, breaking in the third game with a thunderous backhand down the line that drew enormous cheers from the partisan crowd.
But the frustrated 20-time Grand Slam winner could not consolidate his position, losing the next game as Zverev levelled at 2-2.
The eventual tie-break went with serve until the 10th point, when Federer netted to hand Zverev a mini-break -- an advantage he did not squander.
Zverev, who already has three Masters series titles under his belt in his short career, is the youngest finalist at the event since Juan Martin del Potro in 2009 but has been found wanting at the Grand Slams.
In Saturday's later match, world number one Novak Djokovic takes on South Africa's Kevin Anderson in a repeat of the Wimbledon final for the right to face the German in Sunday's final.