Pompey saw their winning run come to an end as they were held to a goalless League One draw at Cambridge.
The best chance they had to leave the Abbey Stadium with all three points was when Abu Kamara’s cross was deflected against the woodwork.
But the result ensured the table-topping Blues maintained their unbeaten start to the season.
John Mousinho made three changes from the side that dramatically beat Carlisle on home soil at the weekend.
He made switches in both full-back positions, with Joe Rafferty and Jack Sparkes making way for Zak Swanson and Connor Ogilvie.
Christian Saydee also returned to the forward line – playing just behind Colby Bishop – as Tino Anjorin dropped to the bench.
Former Pompey duo Michael Morrison and Gassan Ahadme started for the hosts, while Adam May was named among the substitutes and Brandon Haunstrup missed out through injury.
There was a minute’s applause before kick-off in memory of the great Sir Bobby Charlton, who passed away on Saturday at the age of 86.
The visitors were forced into an early change when Ogilvie hobbled off the pitch to be replaced by Sparkes.
And their opponents were the first side to serious threaten, when Sullay Kaikai’s cross drifted towards the goal and had to be tipped over by former United keeper Will Norris.
It was, overall, a fairly even start to proceedings, though, with the game being played at a high tempo, but with some quality lacking in the final third.
Pompey had a couple of penalty appeals turned down by referee Declan Bourne, with Kamara and Swanson both denied.
They began to dominate possession as the first half progressed and were looking the more likely side to break the deadlock.
And that almost happened when Kamara’s delivery deflected off the foot of Liam Bennett and flew against the post.
The ball bounced back towards Bishop, who was denied another goal by a superb Ryan Bennett block.
Conor Shaughnessy then found himself in an unfamiliar position on the right wing and whipped in a fine cross that was just over the head of Bishop.
Pompey went close again just before the break after Jack Stevens failed to get a strong fist on Alex Robertson’s corner.
A scrambled ensured before Shaughnessy fired the ball towards goal, where it was deflected over the bar.
Half Time: Cambridge 0 Pompey 0
The visitors applied plenty of pressure when the action resumed, but a Joe Morrell effort was blocked, while Saydee could only help on a dangerous Sparkes delivery.
Swanson then beat his man on the right and cut into the box before angling in a shot that was saved by Stevens.
Mousinho was forced into another change just before the hour mark when Saydee went down for the second time in quick succession and was replaced by Anjorin.
Pompey were still working hard to find a breakthrough and a Swanson attempt was blocked after Cambridge failed to properly clear a corner.
Paddy Lane was next to try his luck, cutting side from the right and hitting a fierce strike not too far over the bar.
But Cambridge almost took the lead against the run of play on 73 minutes – although it would have been a controversial goal, with Morrell appearing to be fouled by James Brophy in the build-up.
Substitute Saikou Janneh sent a low ball towards Jack Lankester in the box, only for Norris to deny the midfielder at point-blank range.
It suddenly felt like the tide was beginning to turn, as United began to cause problems and the Blues looked a bit ragged.
And while the visitors were still trying to get forward, their opponents were dangerous on the break.
Kusini Yengi relaced Kamara to provide some fresh legs for the closing stages, but it was his fellow striker who had a chance to win the game in stoppage-time.
He was picked out by a Robertson corner at the back post, but his header was punched away by Stevens.
Cambridge (4-1-4-1): Stevens; L.Bennett, R.Bennett, Morrison (c), Andrew; Cousins; Brophy (Rossi 90+7), Digby, Lankester (Thomas 90), Kaikai (Janneh 69); Ahadme
Subs not used: Mannion, May, Gordon, Okenabirhie
Pompey (4-2-3-1): Norris; Swanson, Poole, Shaughnessy, Ogilvie (Sparkes 8); Robertson, Morrell (c); Lane, Saydee (Anjorin 58), Kamara (Yengi 90); Bishop
Booked: Kamara, Morrell, Sparkes, Shaughnessy
Subs not used: Schofield, Rafferty, Towler, Stevenson
Referee: Declan Bourne
Attendance: 6,627 (1,470 Pompey fans)