Matt Harrold hit the only goal as Crawley continued their fine home run with a 1-0 victory over Cambridge.
Harrold's fifth goal of the season in the first half made it seven wins out of their last eight home league games for the Red Devils.
Cambridge right-back Darnell Furlong, making his debut on loan from QPR, came to his side's rescue early on with a well-judged tackle to rob Harrold on the edge of the area.
Keeper Will Norris, called up for his first appearance of the season, parried a clever cross from George Smith before the visitors threatened through Rory Gaffney.
The striker raced through to collect a pass by Barry Corr and was one on one but was denied by veteran keeper Daryl Flahavan.
Midfielder Harrison Dunk later had a shot blocked before Conor Newton put the ball into the side-netting.
Harrold, who had an unsuccessful loan stint at Cambridge last season, struck to put Crawley ahead nine minutes before the break from close range after Lee Barnard drove in a low ball into the six-yard box.
Cambridge midfielder Luke Berry went close to equalising shortly after the break when his rasping 20-yard shot cannoned back off the post.
Gaffney and Newton later both put shots wide for Cambridge but, despite greater possession, United failed to create clear-cut opportunities.