Luke Berry netted inside the first minute as Cambridge moved to the top of the Conference Premier after victory against Forest Green.
A mistake from keeper Sam Russell allowed the midfielder to sweep the ball into an unguarded goal after just 56 seconds.
Richard Tait hit the crossbar before Tom Champion headed the U's second.
Rovers replied when Yan Klukowski pounced on keeper Chris Maxwell's error to tap in but it was not enough.
VIEWS FROM THE DRESSING ROOM
Forest Green Rovers manager Dave Hockaday told BBC Radio Gloucestershire:
"It's two individual errors that's given them the win and that's disappointing, but you can't do a great deal about individual errors.
"We had a go and left ourselves wide open at the end because we just wanted to go and get something from the game and on another day we would have got it.
"It's disappointing and we shouldn't be losing two games on the bounce, and we were certainly good enough to get something today.
"We've got to recover, you're only as good as your last game. We've got to get ourselves mentally and physically right for Tamworth on Tuesday."
Cambridge United 01 Maxwell 03 Taylor 04 Coulson 06 Miller 14 Tait 07 Donaldson (Hughes - 76' ) 08 Champion 11 Dunk 18 Berry 09 Cunnington Booked 21 Appiah (Arnold - 65' ) Substitutes 13 Norris 22 Spillane 24 Arnold 17 Hughes 19 Sam-Yorke Forest Green Rovers 01 Russell 03 Stokes Booked 05 Turley Booked 06 Bennett 15 Oshodi 04 Bangura 23 Barry 09 Wright (Vieira - 67' ) 14 Taylor (Klukowski - 46' ) 18 Barnes-Homer Booked 30 Mangan (Norwood - 55' Booked ) Substitutes 13 Bulman 02 Hodgkiss 08 Klukowski 07 Norwood 10 Vieira