Gustavo Caballero struck his first goal for Portsmouth as they picked up another valuable away win in the Sky Bet Championship with a 3-1 victory at third-placed Millwall.
It is the second time this week that Pompey will be travelling back from south London with all three points - having done so by the same score at Charlton on Tuesday - as they moved six points clear of the relegation zone.
They also completed a double over Millwall this season and dented their hosts' promotion hopes, although teams dropping points around them means not too much damage has been done.
Portsmouth came close to taking the lead after just six minutes when a shot from outside the box by Caballero needed to be pushed over by Anthony Patterson, who was making his debut in the Millwall goal.
The hosts were slow to get going before a cross by Femi Azeez deflected off Regan Poole, but fortunately for the Pompey defender the ball went straight into the arms of goalkeeper Nicolas Schmid.
Alfie Doughty then fired a free-kick from a promising position - just outside the area - over for the Lions, but this flurry from them still came in the middle of what was a decent opening from the visitors.
The match descended into a scrappy affair before Millwall had their best chance of the first half when Camiel Neghli's cross picked out captain Jake Cooper who put his header straight at Schmid.
The teams went in for half-time still deadlocked, but this only remained the case for 19 seconds after the restart.
It was Portsmouth who broke through as Caballero sped past Cooper, with his first shot being saved by Patterson before the Paraguayan squeezed in the rebound from a tough angle.
Pompey's loud away contingent was celebrating again after 55 minutes when Millenic Alli did well down the left before pulling the ball back for John Swift to crash in a first-time effort that doubled his side's lead.
Millwall badly needed a lifeline and they seemed to have one in the 64th minute when Macauley Langstaff laid the ball off for Casper De Norre who delightfully curled in his first goal of the season.
Pompey were straight back on the attack, however, and almost restored their two-goal lead within two minutes when Ebou Adam's header from Swift's corner was tipped over by Patterson.
The hosts failed to heed that warning as they could not clear the resulting corner, with Adams having a shot blocked before Portsmouth captain Marlon Pack found the bottom corner to put his side back firmly in control.
This time, the Lions were unable to force their way back into the picture, as Pompey tasted more success in the capital.
Millwall's Alex Neil:
"I don't think we deserved anything from the game. People will think when I say how we defended that I'm talking about the back four, but I'm talking about the whole team.
"This is a collective thing in that the first goal from our point of view was really disappointing.
"The second goal was equally as disappointing because we're actually 'intact' - the first ball comes up, they nod it down, they win the second ball and work it round the corner, and we tried to nick from the wrong side and let them roll away from us.
"I said to the lads after the game that we're built as a team on doing the basics really well and I didn't think we did the basics well enough today.
"When the ball did go forward, it didn't stick enough for us when we got it there and I didn't think we won enough duels."
Portsmouth's John Mousinho:
"Prior to the new year, we put in a half-decent performance at Derby and we were okay at Swansea, and then obviously at Bristol City we got blown away.
"But after that I think the way that we knuckled down post-Arsenal [in the FA Cup] - I know we lost the Arsenal game, but we were excellent on the day - we've replicated those performances away from home.
"I think it's come from a reset after Bristol City, and a recognition of the fact that we can do both sides of the game pretty well when we put our minds to it.
"To come here, against the third best team in the league, we've watched them time and time again be a very impressive side, home and away from home.
"They've got threats all over the pitch, so to do that today off the back of Tuesday was really pleasing. I thought we were excellent throughout."
