John Terry scored a 93rd-minute winner as Chelsea snatched a 1-0 victory at home to Everton on Saturday that sent them four points clear at the Premier League summit.
Jose Mourinho's side looked destined to drop vital points until Terry marked his return to the team after a three-game absence by toeing Frank Lampard's free-kick past visiting goalkeeper Tim Howard in stoppage time.
"When you score in minute 90-something, you could speak about being lucky, but the reality is that the boys chased it," said Chelsea manager Mourinho.
"I feel sorry for Everton because a point would have been fair, but we were the team that was trying to win, which is our responsibility."
Victory means that Chelsea are assured of first place until next weekend at least, although Arsenal and Manchester City can both trim their lead later on Saturday.
Second-place Arsenal host Sunderland, while third-place City, who now have two games in hand, tackle Stoke City at the Etihad Stadium.
Despite losing January signing Lacina Traore to injury in the warm-up, Everton made a confident start in the sunshine at Stamford Bridge and kept the ball from Chelsea for long periods of the first half.
Leon Osman saw a rising half-volley tipped over the bar by Chelsea goalkeeper Petr Cech, while Phil Jagielka glanced a header wide from a Kevin Mirallas cross.
Howard saw some action either side of half-time, saving from Samuel Eto'o and then Lampard, before a shot from Osman flicked off Mirallas and forced Cech into a smart low stop at the other end.
Howard produced an even better save moments later, parrying a low drive from Eden Hazard and then brilliantly springing to his feet to block a ferocious Branislav Ivanovic attempt with his chest.
Mourinho introduced Fernando Torres and Andre Schuerrle as the hosts pushed for a late breakthrough and saw Ramires lash a 25-yard shot narrowly wide in the 90th minute.
Time looked up for Chelsea, but in the third minute of injury time Terry threw himself at Lampard's left-wing free-kick and managed to squeeze the ball past Howard.
Everton remain in sixth place in the table, eight points below Liverpool, who currently occupy the fourth and final Champions League qualifying place.
Wayne Rooney will hope to celebrate his new Manchester United contract with a goal when his side visit Crystal Palace later on Saturday, a day after he committed himself to Old Trafford until 2019.
Meanwhile, Felix Magath will take charge of his first game as Fulham manager when the league's bottom club visit fourth-bottom West Bromwich Albion.
The 60-year-old, hired to succeed Rene Meulensteen last week, is the first German to manage a club in the Premier League.