Boro suffered a frustrating defeat in Lancashire as Preston won 2-1 at Deepdale.
A frustrating first half for the Teessiders saw the hosts go in at the break a goal in front.
The response after the break was spirited, with Delano Burgzorg pulling Boro level just seven minutes after the interval.
Michael Carrick's side looked the more likely to go on to claim maximum points and created a number of opportunities, but North End struck the decisive blow at the other end with 12 minutes remaining.
Possession
59%Shots
16Shots On Target
6Corners
4Fouls
63
0
Cards
1
0
Boro named an unchanged line-up from the team which started the 2-0 win over West Bromwich Albion, though January arrival Ryan Giles was included on the bench.
The visitors started brightly, with Finn Azaz seeing a strike deflected into the hands of Freddie Woodman and Burgzorg having an effort saved by the legs of the Preston goalkeeper before the follow-up from Hayden Hackney was blocked.
Ben Doak drove at his man down the right as Boro attacked again, but Woodman did well to push his dangerous low ball to safety.
Preston began to stifle their opponents as the first half progressed and went in front themselves on 28 minutes. Stefan Thordarson received the ball on the left of the area, brought it inside onto his right foot and curled inside the opposite post.
Rav van den Berg blocked well from Thordarson to prevent North End doubling their advantage late in the first half.
PNE: Woodman, Storey, Gibson, Hughes, Kesler-Hayden, Thórdarson (Ledson 60'), McCann, Meghoma (Brady 61'), Greenwood (Holmes 72'), Frökjaer-Jensen (Keane 85'), Osmajic (Riis 72')
Subs: Whatmough, Lindsay, Keane, Riis, Brady, Cornell, Ledson, Holmes, Potts
Middlesbrough: Glover, Ayling, Fry, van den Berg, Neto Borges (Giles 74'), Morris, Hackney, Doak, Azaz (Gilbert 81'), Burgzorg, Latte Lath (Forss 11')
Subs: Barlaser, Clarke, Gilbert, Forss, Giles, Connor, Finch, Bailey Palmer, McCabe
Boro returned for the second half with renewed vigour and drew back on level terms within seven minutes.
Doak helped the ball on for Burgzorg and he finished superbly, twisting and turning to find the best shooting angle before firing past Woodman.
The Preston goalkeeper then did well to deny Doak an outstanding solo goal after the young Scot slalomed through the centre of the pitch, and also tipped a sharp Marcus Forss flick over the bar.
Forss was kept off the scoresheet again on 64 minutes when his goalbound shot was brilliantly blocked.
Giles was introduced for his second Boro debut with just more than quarter-of-an-hour left and threatened an immediate impact, with his effort from long range fizzing wide.
Preston had stemmed the regular flow of Boro attacks by that stage and hit their opponents with a sucker punch 12 minutes from the end.
After Tom Glover saved with his shoulder from substitute Emil Riis, the hosts kept the ball alive and the striker was presented with another opportunity which he finished clinically into the bottom corner.
That proved to be the final piece of goalmouth action as Boro were beaten in Lancashire.