Another exhilarating display by young striker Liam Marum, who scored a goal and made another, inspired United who raced into the lead in the 13th minute and were in front again early in the second half.
But each time Altrincham, riding their luck at times, hit back to equalise and they had the tenacity in the last quarter-of-an-hour to hold on for a deserved draw.
Nineteen-year-old Marum followed up his previous performance at the Abbey Stadium, when he was hailed by the fans as an exciting discovery, with an excellent strike as he burst through and coolly chipped the ball over the head of goalkeeper Richard Acton.
And nine minutes into the second half, when United needed a lift after seeing their lead slip away, he produced some acceleration which startled the full-back and led to fellow striker Marcus Richardson finishing off a well-worked move.
Such quality as well as their extra experience and full-time status really should have given United too much of an advantage over the part-timers from Cheshire, but they paid dearly for lapses of concentration in each half.
Altrincham's first goal in the 35th minute came from a highly controversial penalty when defender Rob Gier was adjudged by referee Mike Bull to have handled the ball.
Stephen Aspinall blasted the spot-kick past goalkeeper Paul Crichton giving the visitors such a boost that they went close to taking the lead in their best spell just before half-time.
The ball was ricocheting around the crowded six-yard box in the 38th minute and, when Joe O'Neill got a foot to it, it seemed a certain goal before striking the keeper on the shins and bouncing away.
Altrincham went in looking confident at the end of the 45 minutes, but Cambridge quickly regained the initiative when Marum and Richardson combined to make it 2-1 not long after the restart.
For ten minutes or so after the goal Marum threatened to take control and put United out of sight, but a header arched over the crossbar and a shot was blocked for a corner by the keeper.
Then, out of the blue, as Cambridge eagerly pushed up in search of the goal which would have sealed it, they were caught out completely by a long ball down the right-hand side.
As Altrincham's Robbie Lawton chased it, keeper Crichton seemed to run out of his goal much too early and, when Lawton got to the ball first, he was able to slide in a shot which two defenders racing back could not prevent rolling over the line.
There was only one real incident after that when Cambridge looked like going into the lead for the third time as Richardson blasted a shot from ten yards which was blocked on the line by centre-half Karl Munroe with the keeper beaten.
Cambridge might have been expected to have piled pressure on the part-timers in the final quarter-of-an-hour, especially as they were being cheered on by an excellent midweek crowd of more than 2,500.
But there was a blow when Marum's exploits took a toll on the youngster's fitness and he had to be substituted, robbing the team of much of its cutting edge.
In fact it was Altrincham, backed by their tiny band of fans, that almost snatched what would have been a shock victory from a free-kick six minutes before the final whistle.
With almost every outfield player crammed into the Cambridge area, the kick from Warren Peyton flashed across the box somehow missing every boot as well as the far post by inches.