Darren Bent scored on his Aston Villa debut as Manchester City's Premier League title hopes were hit by a 1-0 defeat at Villa Park.
Bent tapped home in the 18th minute after Joe Hart could only parry Ashley Young's effort, and his strike proved to be enough for the Villans.
With their £59million strike force of Carlos Tevez and Edin Dzeko, the visitors struggled to find a way past Villa's rock-solid defence in which James Collins and former City centre-back Richard Dunne were outstanding.
And the result means City are now three points behind leaders Manchester United having played two games more. Villa, meanwhile, are three points above the bottom three after holding on for only their second win in 11 league games. It also gave them revenge for a 4-0 mauling at Eastlands last month.
Villa had a let-off after 16 minutes when an inswinging corner from Aleksandar Kolarov caused problems but neither Dzeko nor Tevez could apply the decisive touch from close range.
And Villa Park erupted after 18 minutes when Bent converted his first chance for his new club. Gabriel Agbonlahor found Ashley Young in space and his curling effort was parried aside by Joe Hart but Bent was quickly onto the rebound to slot in.
City tried to retaliate and Brad Friedel produced a fine save to tip Vincent Kompany's header over the bar from Kolarov's corner.
Ciaran Clark then became the first player to be yellow-carded after bringing down Dzeko.
Young almost picked out the late run of Bent with a fine pass shortly after but the ball skidded off the surface. Hart was also alert to cut out a similar ball from Agbonlahor aimed at the former Sunderland player.
Former Villa captain Gareth Barry went close to bringing City level after 37 minutes when his glancing header slid past the far post from Jerome Boateng's threatening ball into the box.
Young got plenty of power behind a low 20-yard drive in the last minute of the half but Hart had his body behind the ball.
City continued to press at the start of the second period and Collins blocked a shot from Dzeko after the striker had cut inside following good play by David Silva.
Dzeko then got on the end of a corner from Kolarov but could not keep his header down.
Mancini made his first change after 57 minutes with Adam Johnson replacing Barry who was jeered by home fans as he left the pitch. Soon after Collins again did well to block a drive from Tevez.
But Friedel knew little about keeping out a powerful low drive from Boateng which bounced off him and away to safety.
Dzeko looped a header past the post and the pressure increased from the visitors while Hart was called into action at the other end to turn a dipping drive from Young away for a corner.
Kolarov was booked after 69 minutes for bringing down Marc Albrighton.
Cuellar threw his body in front of a drive from Dzeko to typify Villa's fighting spirit, while Collins produced another superb block to deny a Tevez volley as the game entered the final 10 minutes.
Clark deflected a 25-yard shot from De Jong against a post with Friedel stranded, and then Dzeko headed wide from close range when it seemed easier to score.
In a rare Villa attack Young poked the ball wide at the near post from Albrighton's cross, but one goal was enough.