It all started with a smile.

As Borussia Dortmund defender Nico Schlotterbeck got into position to mark his Bayern Munich counterpart Dayot Upamecano ahead of an early corner kick, the two exchanged a few words.

The Frenchman grinned and laughed softly. Niceties observed, he then evaded Schlotterbeck, powered into the box and headed the ball into the back of the net.

"He just spoke to me," Upamecano revealed to DW afterwards. "He said, 'are you ok?' So I said, 'yes, great, you?' And then I went. And I went fast. I did everything I could to score that goal."

It was a remarkable start for Bayern, setting them on their way to a crushing 4-0 win, Harry Kane netting a second consecutive Bundesliga hat trick as Bayern bounced back from their calamitous German Cup exit against third-division Saarbrücken midweek.

Upamecano gamble validates Tuchel

"Not bad for a team showing no development and with a bad relationship between the coach and the players," scoffed head coach Thomas Tuchel in the post-match press conference, a scathing reference to criticism leveled at him by former-Bayern-players-turned-television-pundits, Lothar Matthäus and Dietmar Hamman, in recent days.

And it was an even more remarkable start for Upamecano who, having missed the last five games with a thigh injury, only started at the Westfalenstadion because Tuchel had no other choice, Matthijs de Ligt having also been ruled out. It was a risk but it paid immediate dividends.

"I felt great," Upamecano told DW. "It was planned that I would come off in the second half but I felt good. I'm proud of myself and of the doctors and the work they've done with me. Now I'm ready for a full 90 minutes."

By the time he was withdrawn after an hour, Bayern were two goals up and cruising – and had been for 50 minutes thanks to two players in flying form: Kane and Leroy Sane, who ran Borussia Dortmund ragged all evening.

The Leroy Sane show

It was Sane's corner which Upamecano had powered home for the opener but that was just the start. Five minutes later, the German international launched a Bayern counter-attack with a neat back-heel and then, as Leon Goretzka stormed through the absent Dortmund midfield, Sane sprinted up alongside him on the overlap and crossed for Kane to tap home.

"It's obviously a huge plus, having players of that quality," said Upamecano of his offensive teammates. "Our attack is so strong – and don't forget Kingsley [Coman] either. He also had a good game today."

But this was the Sane show. The next time he got in behind, Dortmund goalkeeper Gregor Kobel stood tall. Then, he danced past Marius Wolf before wrongfooting Marcel Sabitzer. At one point, he was surrounded by five yellow shirts on the halfway line, but didn't lose possession.

"We know that we have qualities which are hard to defend against," said Tuchel. "The two early goals gave us important stability but then we had a good mix of deep possession and quick attacks."

Bayern should have made it three just after half-time when the irrepressible Sane ruthlessly disposed Salih Özcan and bombed forward. But for once, his final pass let him down. Now it was Kane's turn.

Harry Kane's hat trick of hat tricks

Pre-match, Dortmund head coach Edin Terzic had insisted that there would be no special measures to deal with the England captain, that Dortmund would attempt to deal with him "collectively" and not man-mark.

But it didn't work as Kane consistently dropped deep and wide into those spaces which only he sees, linking up play and then joining in attacks himself before finishing clinically in the box twice more to complete an astonishing third hat trick of the season already.

"It's just the experience and all the know-how he's accumulated throughout his career," Bayern captain Manuel Neuer told Sky. "He didn't leave it all in England; he brought it with him to the Bundesliga."

There was praise from the boardroom, too. "As someone who was partly responsible for this transfer, I'm very happy to see it work this way and I hope it continues," said Bayern chief executive Jan-Christian Dreesen. There was also acknowledgement that the summer's inadequate transfer activity has left the squad down to the bare bones.

"If we consider it necessary to strengthen in some positions, we will do so," he told Sky. "We have a top quality squad, but that doesn't mean we don't want to become even stronger."

Upamecano, as a prime example, was replaced by 19-year-old Aleksandar Pavlovic, who went on to set up Kane's third goal and cap off a perfect evening for Bayern.

"It's not easy to win here but we played brilliantly today," concluded Upamecano. And he was still smiling.