Manchester City, Manchester United and Spurs all failed to win as Leicester take advantage.
There were eight games played on Saturday, and more than enough talking points to work through as some of the traditionally big powers in the league struggled badly.
Chelsea are floundering perilously close to the danger zone and with both Manchester sides in difficulty, the Premier League title race might be more open than it has ever been this year. Three points separate the top four, and with Jurgen Klopp's Liverpool on the up and yet to play, anything could happen.
Taking up that theme, since anything can happen in the world of football, then the most pertinent question at this point of the season is 'when do we start taking Leicester seriously?'.
If it's not Jamie Vardy tearing up defences, it's Riyad Mahrez who steps in, and the Algerian was on hand with a hat-trick to help his side beat Swansea 3-0 at the Liberty Stadium on Saturday afternoon.
Image: Nick Potts / PA Wire/Press Association Images
If it wasn't for Jamie Vardy, Mahrez would be getting a lot more attention in this Leicester set up. He has scored 10 league goals in 14 appearances so far, and Ranieri's team do not appear to be slowing down. Their blend of attacking football is a joy to watch, and if they can get over the Christmas hump in the top four, then it might be time to start considering that they could really finish in the top four at least.
Manchester United had 21 shots to West Ham's nine, but that didn't prevent the fans from booing on the final whistle as they were held 0-0 at home to a side that United supporters believe they should be beating.
Louis van Gaal was without the services of Wayne Rooney and started Marouane Fellaini as his number 10, with Anthony Martial playing up front in Rooney's place.
Image: Dave Thompson / EMPICS Sport
Many believed that not having the United captain available would mean a more fast-paced United attack and plenty of goals. That was not to be the case, however, as Louis van Gaal faces into a tough week that could decide his side's Champions League fate for the season.
They play Wolfsburg on Tuesday, with a loss putting them in real danger of being eliminated from the competition.
Manuel Pellegrini said after his side's comprehensive defeat to Stoke that he has a limited squad for what they are trying to achieve. He said it without irony, but that was not lost on social media.
Remarkable that a manager who had £140m spent in summer, at one of wealthiest clubs, can complain of squad thinness.— Miguel Delaney (@MiguelDelaney) December 5, 2015
Yaya Toure, Sergio Aguero, Samir Nasri, Vincent Kompany and Pablo Zabaleta are all on the injury list, but City still have the Champions League to contend with and a title challenge that looks like it will go down to the wire.
It now appears that the question will be who will they buy in January, rather than if they will spend at all. They will surely be looking for a striker as Wilfried Bony has just not cut the mustard at the Etihad, while Aguero's injuries just keep adding up. It looks as though Jim White will be working overtime as some serious activity in the window is now almost a certainty.
You could just as easily insert Garry Monk's name into this headline as the pressure grows on the Swansea boss. However, with Chelsea now sitting just three points above the relegation zone, there can't have been many collapses as spectacular as that which the reigning Premier League champions are suffering through at the moment.
Whether fans of Mourinho want to hear it or not, Chelsea might have to start considering the fact that they are close to getting involved into a relegation battle with the likes of Sunderland, Bournemouth, Norwich and Swansea for competition.
They will almost surely strengthen in January due to the fact that some of their best performers from last year have disappointed coupled with the fact that there has been some dissent in the camp, not least from Diego Costa. Costa was spotted out until ungodly hours of the morning in a London nightclub just days after being seen to throw a bib towards Mourinho in an act of frustration.
However, if Chelsea are to purchase more players in January, the question that remains to be answered is whether Mourinho is still at the club at that stage.
With 15 games gone, few people would have predicted that the Premier League table would have Leicester at the top and Chelsea fighting near the bottom.