A prolific partnership. A close friendship. A new rivalry.

Best friends Erling Haaland and Jadon Sancho will face each other for the first on Sunday afternoon.

For 18 months or so, they were considered perhaps the deadliest partnership the Bundesliga had seen for many years, but now they will be in direct competition with each other in the 188th Manchester derby.

Haaland, the boyhood City supporter, versus Jadon Sancho, formerly a City Academy player.

Sancho decided his future lay away from City and, in 2017, he left for Borussia Dortmund after just a couple of seasons in sky blue.

The transfer didn’t court major headlines, with the 17-year-old winger yet to play a senior game.

At that stage, Erling Haaland was relatively unknown outside of Norway, with the teenage striker only just starting to score goals at Molde.

By 2018, both Sancho and Haaland were starting to appear on the radar of clubs around Europe.

Dortmund had gently eased Sancho into their first team with just 12 appearances during the 2017/18 campaign.

Haaland scored four in 20 games for Molde in 2017, and then 16 in 30 appearances in 2018.

That was enough for Red Bull Salzburg to take a punt on the teenage Norwegian who joined the Austrians in January 2019.

By the end of 2019/20, Sancho and Haaland were considered two of European football’s most promising talents – Haaland after hitting 28 goals in 22 matches for Salzburg, and Sancho having scored 20 goals in 44 matches for Dortmund.

Within the space of a fortnight that season, Haaland and Sancho won their first senior international caps.

And Dortmund beat off a host of clubs to bring Haaland to the Bundesliga by signing him in January 2020 and following the German winter break, he was named among the subs for the trip to face in-form Augsburg.

Dortmund fell 3-1 behind before Haaland was introduced on 56 minutes – and it took just three minutes for Sancho to thread a ball through to his new team-mate who made the most of the opportunity, hitting a left-foot shot in off the foot of the right-hand post.

The pair had immediately demonstrated an almost telepathic understanding from the word go, and by the end of the game, Haaland had marked his debut with a 20-minute hat-trick as Dortmund spectacularly rallied to win 5-3.

By the end of 2019/20, Haaland had scored a total of 44 goals in 40 games for Salzburg and Dortmund.

The deadly duo would become virtually unstoppable in 2020/21.

That campaign, Sancho’s 38 appearances yielded 16 goals and 20 assists, while Haaland – the beneficiary of many of those Sancho assists, bagged 41 goals in 41 games and assisted 12 more himself – together they were responsible for 57 goals and 32 assists that season – incredible statistics.

Their final appearance together would be the DFB-Pokal final (the German equivalent of the FA Cup) against RB Leipzig, with Sancho and Haaland destroying the opposition with two goals each in a 4-1 win – Sancho assisting Haaland’s second on 87 minutes.

It was a fitting way to end the prolific partnership, and the pair – born just a couple of months apart in 2000, and just 200 miles apart in Leeds and London respectively – were by now close friends.

"Good luck on your next journey bro, we had some good times together! Can't wait to see you shine!" posted Haaland on social media when Sancho’s deal to Manchester United was confirmed in 2021.

Sancho responded: "A connection like no other, we will soon meet again brother. Take care and good luck to you ,too."

“Me and Jadon understood each other very well and he’s a top player,” Haaland told the media afterwards.

“So, of course it’s sad that he goes, but that’s how it is and that’s football. You never know what will happen.”

Little could Sancho have known that within a year, Haaland would be playing in Manchester as well, but in the colours of the Reds’ cross-city rivals.

That meant the friendship has been able to continue and when he signed for the Blues, Haaland was questioned about his old Dortmund pal being in the same city.

“He (Sancho) plays for the other lot (Manchester United) now though, so maybe we’ll have to meet in secret!” he smiled.

Haaland and Sancho’s close bond provides a fascinating subplot for Sunday’s derby.

Of course, as the old saying goes, all friendship will be put aside for 90 minutes and whoever triumphs, expect them to let the other know on social media afterwards.

But this is a game Haaland has looked forward to all his life.

When asked which team he wanted to play against more than any other, he replied,  “Look, I don’t like to say the words but Manchester United."

As a City fan in Norway, he was in a minority with many of his fellow countrymen following either the Reds of Liverpool or Manchester.

He aims to make the most of his first derby…

“There are a lot of United fans in my home town in Norway, so I’ve always had to have a go at them. I’m used to this, and I love it,” he said in a recent FourFourTwo interview.

“Don’t take things too seriously. I want to be able to talk a bit like the fans, otherwise it gets a bit boring, doesn’t it?

“It’s all about banter and I enjoy that. I’ve already met some United supporters here in Manchester, and we’re always joking with each other. It’s about not taking everything too seriously.”

That is, of course, until kick-off on Sunday…