There were a few questionable challenges from both sides, while the hosts were reduced to 10 men after the break.
But despite dominating possession, the Blues were unable to celebrate their long-awaited return to action with a win.
Danny Cowley made one change from the side that had beaten Morecambe on home soil 23 days earlier.
Joe Morrell returned to the centre of midfield, with Arsenal loanee Miguel Azeez dropping to the bench.
It was the home side who were first to threaten after Sean Raggett had intercepted a low Smith delivery.
But the ball fell at the feet of Joe Ironside, who fired in a shot that was beaten away by Gavin Bazunu.
Tempers then rose following a clash between Marcus Harness and Jack Iredale, which infuriated the home side.
Pompey’s forward was put in a headlock by Paul Digby, but referee Joshua Smith let them both off with a talking to before booking Smith for dissent.
The visitors soon had a chance to take the lead, only for Reeco Hackett to fire over after being picked out in the box by Harness.
Cambridge immediately broke down the other end and Bazunu flung himself to the right to palm away a strike from Blues academy graduate Adam May.
It looked as though James Brophy was going to tuck away the loose ball, but Mahlon Romeo dived in to make a fine block.
Pompey kept probing as the first half progressed and were just lacking a quality final ball to truly trouble their opponents.
And it was United who almost went ahead in stoppage-time, but Smith was unable to beat Bazunu from a tight angle.
Cambridge United 0
There was a healthy crowd inside the Abbey Stadium, although they were not treated to much excitement once the action restarted.
Pompey threatened from a couple of corners, while there were some dangerous counter-attacks from the hosts.
But neither side was looking particularly likely to break the deadlock as the contest meandered along.
Things certainly livened up on the hour mark, however, as Smith caught Ogilvie and left the defender with a nasty cut on his face.
It was enough to earn the midfielder a booking from the referee and, following his earlier transgression, a red card.
The action was then halted while medical teams dealt with an emergency among the home supporters.
Both sides departed down the tunnel while treatment was given and a supporter was stretchered from the pitch to warm applause from the stands.
The game restarted after almost 15 minutes and Cowley made an immediate change, with Michael Jacobs replacing Kieron Freeman.
Pompey were keen to get forward and caused a few scares among the United defence, while Hackett then made way for Lee Brown.
But Cambridge had a chance of their own when Digby’s header from a May free-kick was saved by Bazunu, who then grabbed the loose ball before Joe Ironside could capitalise.
It was the visitors enjoying the territorial advantage, though, and Dimitar Mitov got down low to turn behind Jacobs’ strike.
The hosts thought Pompey should have been reduced to 10 men when Romeo was penalised for a foul.
But the Millwall loanee was not punished further than a free-kick and Iredale instead got booked for dissent.
The resulting free-kick from May was nodded clear by Raggett and Iredale’s follow-up effort flew wide.
That earlier delay meant there were 21 minutes added on and the Blues tried to find a breakthrough.
There was still a danger of more dismissals, though, with a Shaun Williams challenge on Brophy causing tempers to flare once more.
United were doing a good job of holding their visitors at bay, but when a Harness ball ran across the box for Ronan Curtis, he blazed wastefully over the bar.
George Hirst was replaced by John Marquis as Cowley made one final roll of the dice in search of a winner.
But the Blues were unable to find their way through a resolute United back-line and a late Connor Ogilvie shot went over as they had to settle for a share of the spoils.
Cambridge (4-2-3-1): Mitov; Williams, Okedina, Iredale, Dunk; Digby (c), Worman (Knibbs 77); Smith, May, Brophy (Hoolahan 90+18); Ironside
Sent off: Smith (two yellow cards)
Booked: Brophy, Iredale
Subs not used: McKenzie-Lyle, Masterson, O’Neil, Weir, Lankester
Pompey (3-4-1-2): Bazunu; Freeman (Jacobs 76), Raggett (c), Ogilvie; Romeo, Morrell, Williams, Hackett (Brown 81); Harness; Hirst (Marquis 90+16), Curtis
Booked: Romeo, Raggett, Williams
Subs not used: Bass, Downing, Azeez, Harrison
Referee: Joshua Smith
Attendance: 6,832 (1,463 Pompey fans)