Chesterfield manager Paul Cook was thrilled by his team's strong second-half display as they completed the double over Oxford.
A goal in the fifth minute by recalled striker Marc Richards brought them a second successive win, and breathed new life into their faint hopes of making the play-offs.
But it needed a hard-working and determined second-half showing to earn the victory against an Oxford side that had won its previous two matches, and which dominated the first half after falling behind.
Cook said: "I thought we were in complete control in the second half. We offered Oxford very few attempts on goal, and I thought we might have nicked a second goal on the break.
"After initially starting well, and scoring, we sat back in the first half which invited pressure onto us. We weren't happy about that, so we had some good words with the players at half-time. I wasn't going mad at them, I just thought we needed to take the game to Oxford much more, and in the second half we did that, and it paid off."
U's boss Chris Wilder said he could not believe that his team were not well ahead at the break.
"We were so wasteful in the final third in the first half," he said. "That first 45 minutes was possibly the most we've been in control here all season.
"I couldn't believe it at half-time that we weren't out of sight. We had 20 crosses, which is a ridiculous number, but couldn't get on the end of one.
"The game was lost for us in the first half. We needed to get something out of that and we didn't.
"It drives me absolutely crackers because coming off at half-time, that should have been done and dusted."