Alexander Zverev stunned Novak Djokovic to win the season-ending ATP Finals on Sunday, earning the 21-year-old the biggest win of his career.
Six-time Grand Slam champion Boris Becker, who coached Djokovic to six major titles between 2014 and 2016, watched the match as a BBC Sport commentator.
He explains why his fellow German's victory over the world number one is a significant moment.
Alexander Zverev winning the ATP Finals is a moment which the whole tennis world has been waiting for.
For years we have been saying tennis needs new faces and strong new players - and he has proved he is the best of the next generation.
Beating Novak Djokovic in the final was a big upset and a big victory for Sascha.
It was the big match which we were all waiting for from him.
Yes, he has won three Masters 1000 titles before, and has beaten Djokovic and Federer before, but to beat Novak and Roger back to back in one of the biggest tournaments in the world tells him, most importantly, but also the world, that he is going to be the next big thing.
The world saw a new superstar in tennis arrive on Sunday.
What Zverev needs to do to go deep in the Slams
We've been talking about the next generation for a couple of years now and Zverev is the best of the lot.
He showed it last year by reaching the number three ranking in the world and following it up this year by winning another Masters title in Madrid.
He has played consistent tennis throughout the year and will now finish it as number four in the world.
Winning the ATP Finals is a big step for him, now doing it over five sets at the Australian Open in January, and then the other Grand Slams, is where it really matters.
His mental resilience was impressive against Novak, as was his staying power.
After breaking Novak's serve he had to serve out for the first set - and hit three aces. That showed his mental strength.
I was impressed with how he started the match, obviously he was the underdog, he lost in straight sets to Novak a couple of days ago, and he didn't blink.
He held his serve easily - regularly hitting serves up to 140mph and finding the target with a high first-serve percentage - and made Novak start to worry.
He wasn't afraid of getting into the long rallies, Novak seemed to be puffing a lot, a lot of running was done, he seemed to be on a mission.
Nobody can stay with Novak normally in long rallies but Sascha did and that frustrated Novak.
Now he has to do this consistently in five-set matches at the Grand Slams.
Lendl can take Zverev to the top
As the head of German men's tennis I've been mentoring Sascha for the last two-and-a-half years so I have practised and travelled with him many times and know him very well.
He is a curious 21-year-old, he walks through life with an open mind and an open perspective.
Often he has asked me about what it takes to win. I've told him it is one thing winning a match or two and reaching a quarter-final, and another being successful at a major event like the ATP finals and Grand Slam.
I talk to him about the old days, when I won Grand Slams, ATP Finals and was number one in the world, what I did in those certain situations.
So I'm happy he now has Ivan Lendl as a coach because Ivan, who won eight Grand Slam titles, can tell him the same stories I can tell him and brings an understanding of the game to his team.
We have a lot of coaches, and no disrespect to them, but in order to win Grand Slam matches and become number one in the world it is easier to talk to people who have been there and about their own experiences.
Often it is a small margin and when you have someone on the sideline who has been there it has a big effect.
It is very brave for Sascha to ask someone like Lendl to join his coaching team.
It shows you Sascha's motivation and his goals for the future. He wants to win and be the best so he is surrounding himself with the best.
|Zverev's best performances at Grand Slams|
|Australian Open||Third round (2017, 2018)|
|French Open||Quarter-finals (2018)|
|Wimbledon||Fourth round (2017)|
|US Open||Third round (2018)|
Djokovic could dominate next year
Despite losing against Zverev, Novak has shown over the past week - and few months - that he is back to his very best.
There is not much difference between the Novak we have seen recently and the Novak who topped the world rankings for 122 weeks, and held all four Grand Slam titles, at his peak in 2015 and 2016.
To come back after a difficult start to the year and an injury to win Wimbledon and the US Open this year, and become the dominant player in the second half of the year, speaks volumes for his mentality and love for the game.
As his former coach, it is wonderful for me to see that because he obviously still has a lot of good tennis in him.
I think the time he had off earlier in the year rejuvenated him and made him realise how beautiful tennis is and how much he still has to give. He is more mature.
His serving is very good at the moment, he has shortened his swing to protect the elbow injury and that works in his favour.
Can he dominate the men's game next year? Why not. He is playing amazing tennis.