Riis scored his eighth league goal of the season
Middlesbrough's promotion push faltered after they were beaten 2-1 by Preston North End at Deepdale.
Stefan Thordarson's solo strike gave the hosts a deserved lead at the break, and they were good value for it.
When Delano Burgzorg levelled just after half time, it looked as though the visitors would go on to win the gamne.
However, Emil Riis struck a late winner to secure a third win in four for the Lilywhites which lifts them up to 14th, seven points off the play-off spots, while Boro drop to sixth.
Boro were dealt a blow when top scorer Emmanuel Latte Lath was forced off after 11 minutes with an injury and was replaced by Marcus Forss.
Burgzog saw a shot saved by Preston goalkeeper Freddie Woodman before Hayden Hackney was denied with the rebound by Lewis Gibson's fine block.
Soon, though, Preston began to assert themselves as Ali McCann fired just over from the edge of the area on 21 minutes.
Just before the half-hour mark, Thordarson put them in front when his powerful run down the left allowed him to cut inside before curling a shot into the bottom corner.
Seven minutes into the second half, Boro levelled when Burgzorg slotted home coolly after Ben Doak flicked the ball into his path.
Moments later, Forss could have completed a swift turnaround, but failed to keep his shot down from inside the area, and Woodman then kept both Doak and Forss out.
But Preston held on and regained the lead 12 minutes from time through Riis who, having been denied moments earlier, made no mistake with a finish into the bottom corner.
Preston manager Paul Heckingbottom told BBC Radio Lancashire:
"Good game. The right outcome for us and three points. I'm delighted with the performance and the application against a good side.
"I've said before, in possession, Boro are a very, very dangerous team.
"We had a couple of moments where we weren't brave enough in the first half but I would always back us to get moments of control and we did that.
"Second half, it was similar. I felt we got to grips and they scored on the counter, but we were a lot stronger without the ball and had moments with it.
Middlesbrough boss Michael Carrick told BBC Radio Tees:
"Today I thought it was there for us. We started well, defended well and took control for large parts.
"There were a couple of moments where we shot ourselves in the foot. The boys are devastated, I'm disappointed. It is a horrible day to be sitting here with nothing.
"We were really good for a lot of the game; when Delano scored, the emotion was great to be part of. We thought it was coming again and it went to other way.
On Emmanuel Latte Lath: "He felt his groin, how badly and to what extent I don't know as yet, we'll have to wait and see. It was a blow but I felt we dealt with it well and should have won the game."
