Colchester 1-0 Stevenage
Sanchez Watt's winner gave Colchester all three points and ended 10-man Stevenage's unbeaten league record.
Watt scored with a fierce low shot from the edge of the penalty area after racing onto a pass from Jabo Ibehre.
The winning goal came two minutes after Stevenage's Anthony Grant had been sent off for two bookable offences.
The visitors had chances but Colchester goalkeeper Mark Cousins saved a 30-yard shot from Filipe Morais, while Bondz N'Gala hit the crossbar late on.
Stevenage, who suffered their first defeat in 12 League One matches in 2012-13, remain second but are now seven points behind leaders Tranmere, while the U's moved up to 17th with their third win in four league games.
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Colchester United manager Joe Dunne told BBC Essex:
"We should enjoy it. We've played a top side today and we've really worked hard for it. But that's the rewards you get if you work hard.
"We stuck to our game plan, we need to look and learn from when they went down to 10 men. When that happened we started to force the ball a bit.
"What Sanchez did was a fantastic piece of skill and because the game was so tight it was going to be an individual piece of skill that opened it up.
"That's why he's at the club he's at. Sanchez's challenge is to that more often."