Forest Green Rovers were relegated at the end of last season only to be reprieved because of Northwich Victoria's misfortune but have recruited wisely in the summer and look a stronger outfit this term.
The U's fielded only three of the players who finished last season in League Two - Andy Duncan, Dan Gleeson and John Turner - and struggled to adapt to their new surroundings.
Mark Peters headed narrowly wide for the visitors in the 12th minute while eight minutes later Fola Onibuje scuffed his shot a yard past the post.
On a rainy day the play deteriorated and became increasingly scrappy in the difficult conditions.
Rovers scored the winner early in the second half. The U's defence was caught flat-footed and midfielder Paul Wanless found Scott Rendell who headed home.
Rendell almost immediately doubled Forest Green's lead only for Andy Duncan to save the visitors with a last-ditch clearance.
Forest Green were reduced to ten men just after the hour when Abdou Sall stamped on the back of Robert Duffy's heels and was immediately shown the red card.
But despite some late pressure Gary Owers' side hung on to clinch three vital points against a Cambridge side that looks as though it will struggle this season.