The result ended Chelsea's unbeaten run in the Premier League which stretched back to January 20.

Knight scored Villa's first when he rose above Didier Drogba to head home Gareth Barry's corner after 47 minutes.

Villa had to soak up some Chelsea pressure to defend their slender lead but nailed a second on the counter-attack with two minutes to go. Ashley Young broke clear down the left and smashed the ball across the box and Gabriel Agbonlahor diverted it past Petr Cech.

Knight, who completed his move to the midlands from Fulham this week, told Sky Sports: "It's a dream come true to score today, it's been a funny old week. "It's great to get a goal on my debut, I've got all my family and friends down here today and I'm glad to get the win. We stuck at it from start to finish."

Knight's central defensive partner Martin Laursen was key to Villa keeping Chelsea at bay and he believes this victory over one of the league's big four teams will do wonders for the confidence of the side.

"It was fantastic. It's a great result and something I've been really hoping we could do because we've struggled against the bigger teams in this league. We went out there with great belief and we won," the Dane said.

"We defended well, we struggled a bit in the beginning of the first half. We were really determined to win this game and we got a great result.

"We have some great possibilities in this team. We have a lot of good young players and we showed against Inter Milan in pre-season that we have a good team."

Defeat cost Chelsea the chance to regain top spot in the Premier League from Liverpool, the last side to defeat them in the competition at Anfield in January.

But then Villa Park has been a bogey ground for the Blues who have not triumphed there in eight seasons.

Chelsea enjoyed far more possession but they struggled to create clear-cut openings against a Villa team who defended as if their lives depended on the outcome.

Martin Laursen, after an uncertain opening, had an excellent game at the heart of the home defence despite being given a searching examination by Didier Drogba.

There was a desperation about Chelsea's play in the second half and the final whistle signalled mass scenes of joy from the majority of the 37,000 crowd.

Worryingly for Chelsea, Drogba had to be helped off the pitch at the final whistle after suffering what looked like a leg injury.

Chelsea began positively and had strong claims for a penalty when central defender Laursen appeared to wrestle Shaun Wright-Phillips to the ground but referee Mark Clattenburg waved play on.

Although Drogba was operating as the only out-and-out striker for Chelsea, they attacked with purpose and pace in the early stages.

But it was Chelsea keeper Petr Cech who was called upon to make the first meaningful save.

Ashley Young's dangerous ball into the near post picked out Agbonlahor and he spun sharply and had his powerful left-footed drive parried away by Cech.

Chelsea were soon back on the offensive and skipper John Terry was only just too high with a header from Florent Malouda's inswinging corner.

Then Villa keeper Scott Carson was twice called into action to save low drives from Michael Essien and Wright-Phillips respectively.

But Agbonlahor would have been disappointed to send a low attempt straight at Cech with no real power after the ball had rebounded off Alex.

Then the England Under-21 striker out-jumped Ashley Cole to make contact with a Gareth Barry cross but could not direct his header on target.

Wright-Phillips would have been disappointed to drive a shot into the side-netting after cutting back inside Agbonlahor.

Villa began the second half on the offensive and Cech tipped over a curling effort from Young after John Carew had laid the ball back into his path.

Then after 47 minutes Villa fan Knight celebrated his debut by putting O'Neill's side in front.

Barry supplied the inswinging corner and Knight, who last weekend scored an own goal on the same ground in his last league match for Fulham, rose above Drogba to head past Cech.

Ashley Cole tried to keep out the downward header but could only help it over the line.

Jose Mourinho's instant response was to take off Mikel and bring on Claudio Pizarro as he reverted from a 4-5-1 to a 4-4-2 formation.

Nigel Reo-Coker became the first player to be yellow-carded for a challenge on Malouda in an off-the-ball incident.

Predictably Chelsea launched an offensive and Drogba showed great skill to spin past Laursen and race into the Villa box but Mellberg did well to cut out his low centre.

Mourinho then brought on Joe Cole and Salomon Kalou for Wright-Phillips and Claude Makelele respectively.

It was now one-way traffic towards the Villa goal and Terry was only just too high with a powerful header from a Joe Cole corner.

But the Blues were still struggling to create clear-cut openings for all their lengthy spells of possession.

Then with two minutes left Agbonlahor turned home Young's cross from the left flank to settle the issue.