Pompey Women missed the chance to close the gap on Oxford in the National League South table after suffering a 2-1 defeat.
Hayley Bridge’s goal saw them battle from behind to go in level at the break, but United ended up snatching a second half winner at Court Place Farm.
Jay Sadler unsurprisingly stuck with the same side that eased to a 4-0 victory at MK Dons the previous week.
It was the visitors who made the brighter start to the contest and Rebekah Tonks fired narrowly over the bar.
But Daisy McLachlan then threatened against her former club at the other end of the pitch and Hannah Haughton was forced into action.
Haughton soon made a superb save from point-blank range after Georgia Timms had met McLachlan’s delivery from the right.
And Oxford did make the breakthrough on 25 minutes, as Kayleigh Hines’ effort from the edge of the box went in off the post.
The Blues quickly hit back, though, and McDonnell picked out BRIDGE at the back post to open her account for the club.
That seemed to fire Oxford up and Haughton denied Timms, before Danielle Rowe cleared Cheryl Williams’ shot off the line.
Half Time: Oxford 1 Pompey 1
Pompey looked to get on top once the action restart and a 20-yard strike from Katie James was palmed away by Paris Sercombe.
The keeper had to be at her best again just past the hour mark, this time to keep out Eilidh Currie’s attempt from long-range.
Charmaine True was brought on to add some more firepower, but it was United who retook the lead on 75 minutes.
McLachlan was the player to find the target, arriving at the far post to lash the ball past Haughton and into the net.
It almost got even worse for Pompey, but Haughton was able to deny substitute Molly Peters from scoring.
The Blues did their best to find a second equaliser and did come close, with Molly Clark thundering a shot against the crossbar.
Pompey: Haughton; Tonks, Southgate (c) (Younger 89), Currie, McDonnell; Clark, Rowe, Bradley (Bath 79); Widdows, James (True 68), Bridge
Subs not used: Filipe, Rowbotham