An injury-time goal by Wigan striker Emile Heskey put a massive dent in Chelsea's hopes of winning the Barclays Premier title.
Michael Essien had given the Blues the lead in the 55th minute but Heskey's shock equaliser left their title hopes in tatters.
Chelsea are now five points adrift of Manchester United at the top of the table and defeat at Everton at Goodison Park on Thursday night would effectively hand Sir Alex Ferguson's men the title.
Wigan were unchanged with Marcus Bent and Heskey leading the attack while Chelsea coach Avram Grant made seven changes from the side which beat Fenerbahce in the Champions League - including a recall for goalkeeper Petr Cech.
The Czech Republic international played with a special chin guard to protect 50 stitches he required after a training-ground clash with defender Tal Ben-Haim two weeks ago.
But Grant was hampered by the sudden loss of midfielder Frank Lampard, who was pulled out of the side just before kick-off with another illness.
Yet despite the handicap, Chelsea should have been in front in the fifth minute but a cross from Salomon Kalou was headed wide from six yards by Nicolas Anelka.
Five minutes later Wigan goalkeeper Chris Kirkland was alert enough to deal with a through-ball from Essien that threatened to provide Anelka with a goalscoring opportunity.
But Wigan withstood Chelsea's early fire and created a real opportunity of their own in the 19th minute when Wilson Palacios back-heeled the ball and Antonio Valencia forced Cech into a fine save at his near post.
Moments later, a fine turn from Michael Ballack in the Wigan penalty area should have caused the visitors more problems but the German's cross was easily dealt with by Kirkland.
Chelsea were largely disappointing going forward and Anelka was left to chase hopeful long balls that Paul Scharner and Emmerson Boyce dealt with comfortably.
Chelsea almost went in front in the 26th minute when a header from Anelka looked certain to loop over Kirkland and into the net until the Wigan goalkeeper somehow managed to get a hand to it.
But while Chelsea were far from impressive, they continued to pressure the Wigan defence.
In the 34th minute, a corner from Juliano Belletti was headed goalwards by Mikel John Obi only for Josip Skoko to clear off his goal-line.
But Steve Bruce's Wigan side were in no mood to capitulate and it needed a last ditch tackle from Scharner to prevent Anelka from getting on the end of a pass from Essien.
Chelsea were running out of ideas as fast as the home crowd were running out of patience.
There was little to enthuse about Chelsea's performance during 45 minutes when they failed to impose themselves on the game.
Grant, was regularly out of his seat on the bench, but despite his calm cajoling, Chelsea could not raise their game and unsurprisingly were booed off at half-time.
Grant's displeasure with the tepid opening half from his Chelsea side produced a half-time change with Joe Cole replacing the ineffective Florent Malouda.
The move almost brought an instant reward for the home side when a shot from Cole was charged down by the Wigan defence.
Another effort from Cole in the 49th minute failed to trouble Kirkland but the England international's introduction had clearly had a galvanising effect on the home side.
Belletti failed to pick out a Chelsea player when he burst into the penalty area and pulled the ball back from the touchline but Grant's side were slowly turning the screw on Wigan.
The visiting defence was now under siege and captain John Terry thumped the ball against the crossbar when he let fly with a volley from Belletti's 53rd-minute corner.
But two minutes later Chelsea were finally in front and Cole was the architect.
His cross into the penalty area was collected by Anelka who looked to have wasted the chance when he slipped over but the Frenchman cleverly knocked the ball into the path of Essien, who beat Kirkland with a rising drive.
Cole almost added a second for Chelsea in the 77th minute but his fierce, angled drive flew inches wide of the target.
Three minutes from time though Chelsea had masked-man Cech to thank when he pulled off a superb save to deny Wigan substitute Antoine Sibierski.
But worse was to come for Chelsea when Heskey volleyed home a cross from substitute Jason Koumas to leave the gap at the top looking almost too wide for Chelsea to close.
