The 19-year-old Norwegian struck the opener in his side's 4-0 victory at Signal Iduna Park on Saturday as Germany's top tier resumed following a two-month suspension due to the coronavirus pandemic.
Raphael Guerreiro added a brace either side of an emphatic Thorgan Hazard finish to complete the Revierderby rout for hosts Dortmund, who showed few signs of rust.
Haaland suggested his 10th goal in nine Bundesliga appearances was to be expected despite admitting he was not yet fully up to speed.
"Of course I'm not the same. I haven't been playing games for seven weeks, so of course not," he told the Bundesliga's official website.
"But I know I have been working hard this period and I am not surprised [to score], no."
Haaland and Dortmund benefited from a brilliant Julian Brandt display that steered the title contenders to within a point of leaders Bayern Munich.
The attacking midfielder was involved in all four goals, each of them celebrated in unusual fashion as players attempted to keep contact to a minimum.
"You have to find a way to make the most of the situation and have fun," Brandt said.
"What better way to start than with a win? Obviously I'd prefer if conditions were normal, but it is what it is.
"It wasn't a perfect performance, but I'm happy after such a lengthy hiatus."
Brandt was caught by Lars Bender in the second minute of Dortmund's 4-3 loss to Bayer Leverkusen on Saturday and was replaced by Giovanni Reyna at half-time.
The attacking midfielder was filmed hobbling as he made his way out of the BayArena, where Dortmund had been without captain Marco Reus due to a muscle injury that has sidelined him until March.
Dortmund have confirmed Brandt will miss the Bundesliga meeting with Eintracht Frankfurt on Friday, which is just four days before they host PSG at Signal Iduna Park.
"Julian Brandt will not be available for next weekend's match against Frankfurt due to an ankle sprain suffered during yesterday’s match," read a statement published on Dortmund's Twitter account.
Dortmund had been due to hold a public training session to showcase their squad, bolstered by the signings of Donyell Malen and Gregor Kobel, though they had to postpone due to positive tests among the first-team group.
The club initially announced Meunier's COVID-19 results with a statement, before releasing details of Brandt's test on Twitter as they communicated the cancellation of the open training session.
The statement concerning Meunier on the official website said: "Borussia Dortmund defender Thomas Meunier has tested positive for coronavirus. The Belgium international took two tests prior to the training camp in Switzerland and an additional test before departing from Bad Ragaz. All of those tests came back negative.
"Having had several days off, the player has now tested positive before commencing training this week. Thomas Meunier has since been in isolation. We hope that Thomas' symptoms are as mild as possible and that he gets well soon."
Later on Tuesday, Dortmund's official Twitter account posted the news that Brandt, too, had tested positive.
Marco Rose's new side finished third in Bundesliga last term, with a late push securing Champions League football for the 2021-22 campaign.
The German side failed to keep hold of Jadon Sancho, who moved to Manchester United in the transfer window, though they do still have Erling Haaland to call upon for the new season.
Dortmund get their Bundesliga campaign underway against Eintracht Frankfurt on August 14 before facing Bayern Munich in the DFL-Supercup three days later.
Lucien Favre's side secured a second-place finish behind champions Bayern Munich thanks to a brace from Erling Haaland on Saturday, the Norwegian taking his tally to 13 goals in 14 Bundesliga appearances.
Dortmund were dominant during the first half and unfortunate to only lead by one at the break, Leipzig goalkeeper Peter Gulacsi called into action on several occasions.
They had to wait until stoppage time to score again, yet Hummels – who was involved in the flowing move that led to Haaland's opener – was pleased with the reaction from the team after a shock midweek loss to Mainz.
"We played badly against Mainz and wanted to show a reaction," said the experienced centre-back.
"We did that and played an absolute top game. It was one of the best performances of the season and it could have gone even higher.
"Overall, we can be very happy – and happy that we secured second place. We had a good mood in the dressing room before the game and were active on the pitch from the first minute. There was a lot of good stuff there."
Dortmund's title hopes were effectively sunk with a home defeat to Bayern following the resumption of the season in Germany, meaning they will finish as runners up for a second successive season.
"We missed too many points in the first half of the season, but our second half is pretty impressive – Bayern just played an even better one," Hummels said, according to quotes reported by Kicker.
"The title must always be the goal if you have such a team."
Knocked out of the Champions League by Paris Saint-Germain and no longer involved in the DFB-Pokal, Dortmund will conclude their 2019-20 campaign with a home fixture against Hoffenheim next Saturday.
"We wanted to secure second place today and we succeeded," said Julian Brandt, who provided the assist for Haaland's second of the game with a low cross.
"I think we have a team that can take criticism. We are very self-critical and question ourselves. On the other hand, we won so many games that you can sometimes lose a game. In the end, that's human."
