Yang Min-hyeok has now scored in successive games for Portsmouth
Portsmouth edged to a narrow home win to end Middlesbrough's unbeaten start to the Championship season.
The home team, who came into the match without a win in four games, took the lead when Tottenham Hotspur loanee Yang Min-hyeok finished with real quality after the away side failed to clear a couple of attacks.
Boro failed to really threaten the Pompey goal throughout and Jo Bursik was not forced into a save of note.
The Teessiders had dropped down to second prior to kick off after Coventry City's thumping win at Sheffield Wednesday but stay two points clear of third-placed Leicester City.
John Mousinho's men made the better start and came close to an opener when Conor Chaplin's close-range effort was blocked behind for a corner kick.
Pompey did go in front shortly after though, with Yang scoring his second goal this week from Jordan Williams' low centre.
There was little by way of a response from the visitors before the break and Chaplin drove a shot just past the post after the interval.
Boro did push on more as the game wore on but Matt Targett's deflected effort, which flew wide, was as close as they came to a leveller.
Despite starting this weekend in top spot, Boro have now managed just 12 goals from their nine matches, one more than 20th-placed Derby County.
They will look to end their three-match winless run when they host Ipswich Town on Friday, 17 October, while Pompey are back in action the following day with a trip to promotion-chasing Leicester City.
'A brilliant performance' - reaction
Portsmouth boss John Mousinho told BBC Radio Solent:
"We spoke on Thursday about the importance of this game and how it would shape how the start of the season for us has been mentally.
"Getting the win was great, and it feels like we have had a good start to the season. Results matter in this game and today we managed to get the job done.
"My overall thoughts was that it was a brilliant performance and there was loads of improvements from Wednesday night against Watford."
John Mousinho: 'The win caps off a good start'
Middlesbrough boss Rob Edwards told BBC Radio Tees:
"It was frustrating in the end. I'm gutted for the fans that we're not able to take something away and they enjoy the journey back.
"It's been a really good start. Today was frustrating, and we have to take it on the chin and continue with the work that we're doing that is building something.
"We had more shots than them, but not enough, and not enough on target, certainly.
"Once they got the goal, which was obviously avoidable from our point of view, it gave them something to hold on to as well. We weren't quite good enough in the final third, ultimately.
"We chucked as many forwards on the pitch as we could to try and get something, but it didn't happen."
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