First, he hit the net from the spot after winning the penalty himself, and then he just got ahead of Walker to loop the ball home.
The scores remained level until the seventy eighth minute as well, but a late double from Gary Roberts gave Portsmouth the win.
A fantastic double save from Sam Walker on thirty nine minutes saw the U's go in at half time level, but it was the hosts who had dominated the first period.
The U's have been beaten 2-0 at Portsmouth in what was their first league defeat of the season.
It was the first win of the campaign for the hosts, and the first defeat for the U's.
With a strong away following to attack to, the U’s started well and were the first to put the opposing defence under pressure.
Unfortunately, neither Brennan Dickenson nor Denny Johnstone could make the most of the breaks into the final third.
The first chance the hosts got came on four minutes, as Main flicked a boot out but failed to test Sam Walker at his near post.
On twelve minutes, a good move down the left got Pompey in behind the U’s defence, but the low cross didn’t find an attacker and the danger was cleared.
At the other end, Johnstone looked set to head goalwards but Christian Burgess just got a touch to take it away, though the referee gave a goal kick.
As the half wore on, the home side were putting the U’s under more and more pressure and Tom Eastman had to be alert to clear when the ball came into the box on twenty four minutes.
Kyle Bennett cut inside from his right wing spot but having got himself a couple of yards to work in, he dragged his shot well wide from twenty yards out.
Walker calmly collected a cross into his box on thirty six minutes, but had to work much harder to keep the scores goalless three minutes later.
He made a fantastic double save from Main and Rose, before the former smashed the third effort well over the bar when he really should have hit the target.
There were no further meaningful chances in the first half but it was the home side that continued to ask the questions.
It was a half that Portsmouth dominated, but the U’s had defended resolutely and kept the hosts at bay.
Home boss Paul Cook would have wanted his side to test Walker more in the second period, but there was no real evidence of more attacking intent from the hosts early on.
Walker did take a Baker shot at the second attempt on forty eight minutes, but the possession the home side had wasn’t being transferred into clear cut chances.
Bennett hit a free kick well over and wide over the bar but the best chance of the half didn’t come until the sixty ninth minute – and it was at the other end.
Denny Johnstone and Brennan Dickenson combined on the left and the former touched the ball into Craig Slater’s path.
His low shot was heading just inside the post but Forde dived full length to push the ball out and away.
Drey Wright was introduced on fifty seven minutes for Kurtis Guthrie, who had been booked and received another talking to from the referee following another foul.
On seventy minutes, Chris Porter came on for Johnstone as the U’s were starting to get forward a bit more.
The next meaningful action came in the seventy eighth minute, with sub Wright deemed to have bought down Gary Roberts just inside the U’s box.
Roberts took the spot kick himself and drilled it down the middle, with Walker’s touch not enough to prevent Pompey going 1-0 up.
Macauley Bonne was the last throw of the dice for John McGreal, coming on for Slater with eight minutes remaining.
He had only been on the pitch a matter of seconds when Portsmouth made the game safe, with Roberts doubling his and the U’s tally.
A long ball found him on the right, he cut inside Brindley with his first touch and then got a bit of luck with his effort looping into the U’s net off Walker.
The final chance came for Roberts as well, but he shot wide going for his hat-trick in injury time.
It was a resilient performance from the U’s but the hosts managed to secure their first win of the season, and in turn inflict the first defeat on the U’s.