Motherwell, bottom of the Scottish premier league, handed leaders Rangers their first defeat of the season after a James McFadden header gave them a 1-0 victory at Fir Park.
Celtic took full advantage as a John Hartson hat-trick helped them to a 4-2 victory over third-placed Hearts.
Rangers, with 57 points, now lead their Glasgow rivals by just one point. Hearts and Dunfermline, who lost 2-0 at home to Kilmarnock, are both 23 points further back in third and fourth.
Motherwell had won only three previous league games this season, though one of them was against Celtic, and they completed a glorious double when striker McFadden headed in a Steve Hammell free kick in the 65th minute.
Hartson's treble made it six goals in five games for Celtic, who had fallen behind to a Mark de Vries goal at Parkhead after four minutes.
Hartson headed the equaliser in the 22nd minute and fired Celtic ahead a minute before the break. De Vries struck again to equalise in the 66th minute but Hartson completed his hat-trick two minutes later before Henrik Larsson weighed in with his 21st of the season to complete the victory.
"It was a magnificent game. Hearts came and played their part, caused us all sorts of problems but we've scored four goals and I'm absolutely delighted with the efforts of the players," Celtic manager Martin O'Neill told reporters.
"The performance here and the result was so important for us. John Hartson was fantastic and scored a great third goal. Hearts were very positive today but we played some fantastic football.
"There'll be lot of twists and turns between now and the end of the season, but this was a big win because there was no point in someone else doing us a favour if we couldn't do ourselves one."
Rangers manager Alex McLeish was predictably unhappy about his side's first league defeat of the season but praised Motherwell's fighting qualities.
"Give Motherwell credit because I thought they deserved it today and probably had the better chances," said McLeish.
"We had some poor finishing in the second-half but other than that you must give the home team credit.
"We can't make excuses. This is a stadium where you have to come, roll your sleeves up and say if you win here it's possible to win championships, but we've been on a long run and I give the players credit but we have to start again. Today was an under-par performance."