A season dominated by the old guard of men's tennis might just have an interloper at the end of it after Alex Zverev stunned Roger Federer at the 02 Arena.
What was the match of the week to date ended with a mix of cheers and boos, as the 21-year-old German defeated the Swiss legend 7-5 7-6 before a packed crowd to make Sunday's final of the Nitto ATP Finals.
He will face the winner of Saturday night's match between Novak Djokovic and Kevin Anderson after arguably the biggest win of his career.
The German felt the need to apologise on court to the crowd after an incident in the tiebreak when a point was stopped at 3-4 when Federer had hit a down the line backhand because a ball spilled from a ballboy's pocket.
It annoyed Federer, although the rules were only being followed, and he made a horrendous volleying error afterwards to go two match points down and ended up losing the tiebreak 7-5.
'First of all I want to apologise for the frustration of the tiebreak,' said the German afterwards, with on court presenter Annabel Croft admonishing the crowd for booing.
'The ball boy dropped the ball so it's in the rules we have to replay the point.' added Zverev.
Now coached by Ivan Lendl, he has yet to make the last four of a Grand Slam but showed why he has been prolific at the level just under that in a high quality contest that was a significant upgrade on many of the matches earlier on this week.
Especially impressive on his supposedly weaker forehand side, he broke to love to take the first set and then came back from an early break down in the second to force the tiebreak.