The Danish striker got PNE’s winner in the closing stages of the clash at the Riverside Stadium.
Admittedly it was a smash-and-grab job, North End only coming to life as an attacking force late on.
Ched Evans headed them level before Riis made it 2-1 with eight minutes left.
It took his tally for the season to 12 goals, many of them having come from his right boot.
So perhaps it showed Riis’ growing confidence that he trusted his left foot to do the business on Teesside.
A mistake from Boro defender Sol Bamba gave him the shooting chance, one the 23-year-old took full advantage of.
Riis told the Lancashire Post: “I thought at first I was going to shoot with my right but Bamba came across quickly to try and make up for his mistake.
“I’m confident with my left foot so I came back on to it and took the shot.
“Seeing it hit the net was a nice moment and for it to be the winner was a big moment.
“That’s 12 goals for the season, eight of them in the league which is all right. Now I want to reach double figures in the league, that is the next target for me.”
Riis started off with regular strike partner Sean Maguire next to him against Boro.
But it was all change early in the second half as Maguire gave way to Evans up front.
The switch worked well, Evans scoring on his return from a three-month lay-off with a foot injury.
Riis said: “Ched came on and showed his physicality. His header was very good and he did well coming back from a long time out with an injury.
“Sometimes you have to switch things to get back into a game.
“When Sean and myself play together, he likes to sink and link the play.
“Ched is more physical who likes to stay higher up the pitch and frighten defences.
“It’s good to have Ched back available, you need different options.”
Riis’ form this season has seen the door open to international football.
He got his first call-up to the Denmark squad for the recent World Cup qualifier against Scotland at Hampden Park.
Although he did not get further than the bench that night in Glasgow, he is on the radar of the Danish set-up with the World Cup just a year away.
His Danish counterpart at Deepdale, goalkeeper Daniel Iversen, surely cannot be too far behind in terms of a call-up.
Iversen was superb in the win at Boro – without him North End would have been dead and buried long before Riis found the net.
“Daniel has been brilliant for us, Tuesday was not the first time he has kept us in games,” said Riis.
“Middlesbrough must have been devastated not to have killed the game but Daniel was in great form.”
The next assignment for Riis, Iversen and PNE, is Fulham’s televised visit to Deepdale on Saturday (12.30pm). It’s a tough one, with the Londoners arriving as league leaders.
And in Aleksander Mitrovic, they have the Championship’s leading scorer with 21 goals.
Said Riis: “It is a game we are looking forward to.
“They have one of the best strikers around and we know it is going to be a tough game. We have done well against the big teams so we think we have a good chance – that is how we approach it.”
It is 14 months since Riis arrived at Deepdale, last season more about settling in rather than goals.
He signed on October 1 from Randers FC for £1.2m, that coming during the extended summer/autumn transfer window afforded to clubs following the re-start period.
Riis scored three goals last season, two of them in the Championship and the other in the FA Cup.
The improvement has been dramatic, confidence playing a huge part in this campaign’s form.