Alexander Zverev stunned world number 1 Rafael Nadal 6-2 6-4 in the first match of his ATP Finals title defense.
World number 5 beat Roger Federer and Novak Djokovic consecutively to win the trophy at London's O2 Arena last year.
And he sent a statement to the rest of the field, comfortably surpassing the highest seed tonight.
The 22-year-old then credited the British crowd for showing him the love he needed to do his best.
"It's great. Obviously everyone knows how much I was fighting all season and that means a lot to play here again," Zverev said.
"After winning the biggest title of my career here last year, this place means a lot to me.
"I was looking forward to this match and thank you very much to everyone who showed up and supported me.
"That's why, in the environment of the season, everyone is trying to make London.
"Playing here in front of you is something we don't have during the year. This is very special and thank you very much.
"Getting back here to train on Thursday and Friday was amazing.
"Being at the stadium was something I wanted to do this year. It was a big goal of mine."
"There haven't always been many highs so I hope I can play my best tennis this week and we'll see where it goes."
Zverev earned 88% of his first service points during the battle with Nadal and later joked that it was because of the food his mother gave him.
"Carrot, spinach, whatever's coming my way, thanks to Mom for all she was giving me," he added.
"I was listening to my parents this week."
Zverev faces Stefanos Tsitsipas in his second round of robin on Wednesday, while Nadal returns to action against Daniil Medvedev.