Substitute Harry Darling's first professional goal was a late winner as Cambridge came from behind to secure a 2-1 victory at East Anglian rivals Colchester.
The hosts had made a fast start to the League Two contest, taking a ninth-minute lead following a confident penalty by Luke Norris.
But they failed to add a second despite dominating the first half, with Luke Prosser heading against the crossbar from Jevani Brown's corner late in the opening period.
And they were made to rue that miss as the visitors hit back to record their first league victory of the season.
Cambridge were on level terms 10 minutes into the second half as Marc Richards headed home from Leon Davies' cross.
And their winner came four minutes from time when goalkeeper Dean Gerken was unable to keep out an acrobatic effort from Darling.
Colchester are still waiting for their first league victory of the campaign after starting with a draw and two defeats, though they saw off Swindon in the Carabao Cup in midweek.