Struggling QPR began the new year with a crucial victory thanks to Ray Jones' first-half header.
Rangers had lost seven of their previous eight matches in the Coca-Cola Championship and began the day only three points above the relegation zone, but Jones' 36th-minute effort was enough to sink play-off chasing Colchester.
Kevin Gallen hit the post for Rangers and Dexter Blackstock and Jones wasted clear chances, while the visitors enjoyed spells of pressure but rarely tested Rangers goalkeeper Simon Royce.
The teams were playing their fourth match in 10 days, and the fatigue showed in the early stages, with neither team capable of finding rhythm or fluency.
Fans had to wait until the 17th minute for the first shot on target, when Lee Cook breezed past Colchester's highly-rated right-back Greg Halford before firing straight at Dean Gerken from 15 yards.
Rangers came close again in the 21st minute when Gallen found the recalled Nick Ward on the right and the Australian cut inside before unleashing a left-footed drive which Gerken gathered at the second attempt.
Colchester were showing little of the form that had lifted them to sixth in the Championship this season, and Rangers moved ahead nine minutes before the break when Cook's left-wing cross was deflected into the path of Jones, who rose at the near post and guided his header into Gerken's left corner.
Rangers should have wrapped up the game early in the second half, but they hit the post and wasted two simple chances to do so.
Only 13 seconds after the re-start, Gallen's angled volley was pushed against the frame of the goal by Gerken, before both Blackstock and Jones produced remarkable misses.
Pat Baldwin's clearance against Blackstock rebounded favourably for the striker, who charged into the area but delayed his shot for too long, allowing Gerken to stop his six-yard effort.
And in the 57th minute, the unmarked Jones somehow managed to head Cook's perfect left-wing cross high and wide at the far post.
Having survived those three scares, Colchester came back into the match. In the 65th minute, Halford escaped his marker at the near post but could only head Kevin Watson's right-wing corner into the United fans behind the goal.
Six minutes later, Pat Kanyuka did well to block Jamie Cureton's close-range effort after Kem Izzet's mis-directed strike had found its way to the former QPR striker.
Geraint Williams' men went close again when Johnnie Jackson headed fellow substitute Hogan Ephraim's cross wide, but Rangers held on for an important win.
Teams:
QPR Royce, Bignot, Mancienne, Kanyuka, Stewart,Ward (Baidoo 60), Gallen, Lomas, Cook, Ray Jones (Furlong 75),Blackstock.
Subs Not Used: Cole, Rehman, Bailey.
Booked: Lomas.
Goals: Ray Jones 36.
Colchester Gerken, Halford, Baldwin (Guy 85), Brown, Barker,Garcia, Watson, Izzet (Ephraim 73), McLeod (Jackson 63),Cureton, Iwelumo.
Subs Not Used: Davison, Richards.
Att: 11,319
Ref: P Walton (Northamptonshire).