With all his substitutes on, Chelsea boss Claudio Ranieri saw Jesper Gronkjaer limp to the touchline in agony 12 minutes from time as the Londoners searched desperately for a second breakthrough.

But Gianfranco Zola’s free-kick caused mayhem in the Derby box and though Andy Oakes palmed John Terry’s header onto a post and Mikael Forssell saw his follow-up blocked, Petit kept his nerve to volley home from the edge of the area.

It was harsh luck on Derby, and particularly their impressive central defender Chris Riggott.

Terry had given Chelsea the lead just after the break with a bullet header, but second-bottom Derby responded quickly with Branko Strupar squeezing home Warren Barton’s cross.

The visitors looked more likely to grab the winner, particularly with Gronkjaer’s departure.

But Zola remains the Chelsea magician and Derby were left to curse their luck as they trooped off dejectedly.

Eidur Gudjohnsen’s partnership with Jimmy Floyd Hasselbaink has so far produced 49 goals in all competitions, but they didn’t enjoy the best of fortunes this afternoon.

The nearest either man came to hitting the half century was to batter Andy Oakes’ right hand post in the opening period.

It took only four minutes for Gudjohnsen to find the woodwork, steering Hasselbaink’s lay-off away

from the Rams returning keeper but not into the net.

Oakes then tipped Frank Lampard’s looping shot over and saved with his feet from Hasselbaink to prevent Chelsea going in front.

But it was another 35 minutes before he was tested again.

Riggott had marshalled the Derby defence brilliantly and was harshly handed a yellow card when Mario Stanic tumbled under minimal contact.

Hasselbaink hit the free-kick low and hard, and it bulleted back off a past with Oakes again beaten.

The Dutchman had wasted Chelsea’s only other decent opportunity in the opening period, rising unmarked but failing to trouble Oakes with a poor header from Gronkjaer’s cross.

After their early problems, Derby gradually regained their composure, with Riggott particularly impressive.

Liverpool are believed to be one of a number of clubs enquiring about the 21-year-old and on this evidence it is easy to see why.

Cool and assured with the ball at his feet and solid despite his lack of physical presence, the confidence he exuded soared through the visitors, even if their own attacking attempts were somewhat limited.

With Rio Ferdinand looking like an England regular for years to come, Riggott’s international prospects are likely to be challenged by Terry.

Some ill advised drinking binges and off-field scrapes have given the youngster too many headlines for the wrong reasons this term.

But he remains an outstanding young talent, and took just five minutes to impose himself on this contest after taking the field for injured William Gallas immediately after the break.

His first touch was to clear a Derby corner. His second was a close range bullet header from Graeme Le Saux’s cross which powered past Oakes.

After six weeks out with a broken foot, the relief was evident as Terry saluted the home fans celebrating his first Premier League strike of the season.

Le Saux himself would have been quietly satisfied with his contribution.

It would take some impressive performances now to force his way into Sven-Goran Eriksson’s World Cup plans.

England’s Swedish coach has undoubted fears over Le Saux’s temperament. The former Blackburn man was returning from suspension today and marked the occasion with another yellow card for dissent.

But he remains a potent attacking threat and his corner which created the opener was perfect.

Derby had provided only a sporadic threat to the home goal, but were level on the hour.

Fabrizio Ravanelli had put in an impressive stint harrying the home defenders but had contributed little in attack until he took possession on the right touchline.

However, the Italian drew Le Saux closer, then released Warren Barton, whose run into the penalty area had gone unchecked. The Derby skipper squared across the face of goal to Branko Strupar who slid the ball past Carlo Cudicini.

It looked like being a priceless equaliser as Chelsea frustrations mounted.

But Petit remains cool in a crisis and the tumbling Luciano Zavagno could only watch in agony as the Frenchman’s shot struck his chest and floated in.