Pompey’s unbeaten start to pre-season was ended by Sutton on a muggy afternoon at the Borough Sports Ground.
The hosts dominated the first half and deservedly led at the break thanks to goals from Roarie Deacon – who was on trial with the Blues earlier in the summer – and Bedsente Gomis.
But Paul Cook’s side looked much stronger following a host of second half substitutions and grabbed a consolation through Christian Oxlade-Chamberlain.
After a slow start, it was the non-league side who were first to threaten – and they immediately forged ahead.
It was Deacon who opened the scoring on eight minutes, steering the ball into the bottom corner after Dan Spence’s low cross had been laid into his path.
The hosts were, unsurprisingly, looking more comfortable on their artificial surface and caused the Blues back-line a few problems.
Ross Worner was not being overly troubled in the United goal, although he made a regulation save to deny Conor Chaplin following good work by Adam May and Brandon Haunstrup.
Instead it was Sutton providing most of the attacking threat and Ryan Burge swung a free-kick narrowly over the crossbar.
Alex Bass then had to beat away a shot from Dan Wishart after the left-back had beaten Chris Sessegnon and burst into the box.
Gomis was next to go close when he met Deacon’s cross and sent a header crashing against the crossbar.
And it was the midfielder who doubled the hosts’ advantage on 33 minutes, arriving at the far post to convert Ross Stearn’s cross.
Half Time: Sutton 2 Pompey 0
Sutton should have had a third goal early in the second half, but Craig Eastmond blazed over the bar after being picked out by Deacon.
Pompey were still struggling to find a way through at the other end, although Curtis Main was not too far away with an audacious long-range effort that flashed over the bar.
They had another opening after breaking quickly up the pitch moments later, but Ben Close was crowded out and Main’s neat flick landed on the bar.
Main was the pick of the bunch for the Blues and he went close again on 65 minutes, with an effort that was deflected narrowly over.
Jack Whatmough came on just past the hour mark, with a further nine changes following a few moments later as Cook looked to give his whole squad a run-out.
And the visitors immediately looked brighter, with only a brave block preventing Theo Widdrington from reducing the deficit.
Pompey were suddenly providing a real attacking threat, with Carl Baker keen to add to his brace at Havant & Waterlooville during the week.
The summer signing fired a free-kick just past the post and then saw a shot kept out by the feet of Worner.
In between those chances, Gary Roberts was also denied by the Sutton keeper as the Blues tried to reduce the deficit.
And that is exactly what they did on 81 minutes when trialist Noel Hunt played a quick free-kick to Baker, whose low cross was converted by OXLADE-CHAMBERLAIN.
A resurgent Pompey tried to grab an equaliser, but it was last season’s National League South champions who able to celebrate the victory.
Sutton (4-1-4-1): Worner; Spence, Beckwith (John 69), Collins (c), Wishart (Amankwaah 63); Eastmond; Deacon, Gomis (Bailey 75), Burge (Downer 74), Stearn (Hudson-Odoi 66); Fitchett (Dundas 60)
Goals: Deacon 8, Gomis 33
Pompey (4-2-3-1): Bass; Sessegnon (Talbot 69), Burgess (c) (Whatmough 63), Barton (Widdrington 69), Haunstrup (Stevens 69); Close (Rose 69), May (Doyle 69); Tollitt (Baker 69), Chaplin (Roberts 69), Lalkovic (Oxlade-Chamberlain 69); Main (Hunt 69)
Goals: Oxlade-Chamberlain 81
Referee: Craig Hicks