Cambridge held leaders Grimsby to a 0-0 draw in a game full of chances at the Abbey Stadium.
The contest got off to a flying start, with both sides having opportunities to take the lead inside the opening five minutes.
Striker Michael Gash came within inches of connecting with Curtis Haynes-Brown's cross and at the other end, Andy Cook hit the crossbar before seeing his follow-up effort cleared off the line by Josh Coulson.
Sheffield Wednesday loanee Matthew Fletcher, making his Cambridge debut, saw a close-range shot well blocked as United finished the first half on top.
Substitute Richard Brodie hit the woodwork with an ambitious effort which had Craig Ross well beaten after 53 minutes.
Moments later the Mariners had a goal disallowed for offside as Marcus Marshall wheeled away in celebration prematurely.
Derek Niven and Jamie Devitt linked brilliantly to carve out a superb opening for the visitors late on, but the latter's shot flashed just wide of the far post.