Referee Mike Dean was the source of drama at Loftus Road, awarding a controversial spot-kick for a push by Clint Hill on Daniel Sturridge, that sealed victory for the visitors
After a forgettable first half, Mata delivered the telling stroke home from the spot on the 62nd minute to put Chelsea's name in the hat for the fifth round draw. However, the winning goal was shrouded in controversy as referee Mike Dean deemed Clint Hill's push on Daniel Sturridge in the box enough to warrant a penalty, much to the disbelief of the home fans.
The game started brightly with Ashley Cole's crunching tackle setting the tone for the game. However any hopes of a feisty west London clash quickly diminished as Chelsea settled into its stride. QPR found it hard to gain and retain possession as the away side stroked the ball across the Loftus Road pitch.
Mata had the first real chance of the game in the 12th minute, pouncing on Luke Young's slip to drive at the heart of the QPR defense. His shot from just inside the area was on target but close enough to Paddy Kenny to parry it away for a corner.
Chelsea was reduced to shots from range as Fernando Torres drifted wide in search of the ball and it nearly paid off as Raul Meireles found room on the edge of the box to take a touch on his thigh and volley the ball at goal. The well-struck shot fizzed wide of Kenny's right-hand post.
There were moments of magic though and the most notable invariably came from Mata, whose backheel on 40 minutes played Ramires into the right channel of the box. For the Spaniard's good work, the Brazilian fluffed his lines, over-hitting his cross well past the waiting Florent Malouda at the back post.
Spot on | Mata sends Chelsea into the fifth round of the FA Cup at Loftus Road
Halftime led to Mark Hughes making one change, introducing Manchester United loanee Federico Macheda for the largely innocuous Heider Helguson but Villas-Boas' team provided the early threats on goal.
Torres' fine work inside the right corner of the box saw him elude two QPR defenders and feed Sturridge marginally behind him. The young English striker's powerful first-time left foot strike looked to be headed in but rose just over Kenny's crossbar.
Then came the minute of drama. QPR produced their first glimmer of threat as Macheda and Wright-Phillips teamed up to unlock the Chelsea defense. As Cole was left on his heels, Wright-Phillips nipped down the right channel but his flashed drive across goal was beaten away by Petr Cech.
The Chelsea break, perhaps stirred by the QPR chance, showed intent but it was shrouded in controversy. As Mata cut back and crossed towards Sturridge, the striker felt contact from Hill and tumbled in the box. Referee Mike Dean had no hesitation in pointing to the spot amid boos from the home support, who had presumed Sturridge was guilty of simulation.
Regardless, Mata composed himself before striding towards the penalty spot and sending Kenny the wrong way to give Chelsea the lead in front of the away support.
As the away side battened down the hatches, Ramires was stretchered off after tumbling awkwardly on the edge of his box. The time taken to deal with the Brazilian allowed Villas-Boas to compose his team and ensure it saw out a result that was much deserved.
QPR mustered one genuine chance in the dying seconds of the game, Young's low drive across goal was pushed to safety by Cech to maintain his clean sheet and ensure the Blues safe progression.
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