Manchester City's 1-1 draw at home to Hull City allowed Chelsea to establish a seven-point lead in the Premier League by winning 2-1 at Aston Villa on Saturday.
City had kept Chelsea within their sights by holding Jose Mourinho's side to a 1-1 draw at Stamford Bridge last weekend, but after slipping up against Hull, the champions' grip on the title is weakening.
James Milner earned the hosts a share of the points with an injury-time free-kick after David Meyler had put Hull ahead in the first half, but City have now won only once in six league games.
"It is difficult to know what the reason is for the spark going," said City manager Manuel Pellegrini.
"Every point you lose, it is more difficult to recover. But in football you never know. We are not thinking about Chelsea.
"They are six (seven) points clear, but can lose games. We are thinking about our performance."
Pellegrini fielded Edin Dzeko up front alongside Sergio Aguero, but it was Hull, who began the game in the bottom three, who showed the greater enterprise.
Ahmed Elmohamady hit the crossbar with a header from a Robbie Brady cross before Meyler broke the deadlock in the 35th minute, sweeping home after Gaston Ramirez's low shot hit the right-hand post.
After Aguero had hit the bar in the second half, Milner bent in a 25-yard free-kick in the 92nd minute, but it could not prevent City registering a fifth game without a win in all competitions.
Chelsea took full advantage as Mourinho claimed his first victory at Villa Park at the sixth attempt.
Eden Hazard broke the deadlock in the eighth minute, finishing from Willian's incisive pass after Oscar had found space on the right.
The unmarked Jores Okore headed in a 48th-minute equaliser, ending an 11-hour goal drought for Villa, but Branislav Ivanovic restored Chelsea's lead with a powerful left-foot shot in the 66th minute.
"It is one more hurdle in the title race, but there are many more," Mourinho told the BBC.
On City's slip-up, he added: "I like, but that was not my concern today. I was never in the game asking for that result. We have to think about us and focus on our matches."
KANE SINKS ARSENAL
Earlier, local favourite Harry Kane scored twice as Tottenham Hotspur leapfrogged Champions League qualification rivals Arsenal by coming from behind to win a pulsating north London derby 2-1.
Mesut Ozil volleyed Arsenal in front at White Hart Lane, but a ferocious Tottenham onslaught saw Kane net a 56th-minute equaliser before heading in a sensational winner four minutes from time.
The home-grown striker's double, his 21st and 22nd goals of the campaign, took Mauricio Pochettino's side up to fifth place, a point above Arsenal, who fell to sixth.
"The fans were incredible, the atmosphere was like lightning and I will remember this for the rest of my career," Kane told BT Sport.
"My first London derby, and to win the way we did by digging deep, it's incredible. I probably won't sleep tonight."
Ozil converted Olivier Giroud's mimis-hit shot in the 11th minute, but it provoked a wave of Spurs attacks that culminated in Kane tucking in an equaliser after David Ospina parried Mousa Dembele's header.
Kane then settled it at the death, meeting Nabil Bentaleb's left-wing cross with a textbook header to condemn Arsenal manager Arsene Wenger to defeat in his 700th Premier League game.
Spurs briefly crept into the top four, but they slipped back after Southampton claimed a last-gasp 1-0 win at second-bottom Queens Park Rangers, who are without a manager after Harry Redknapp's resignation.
Senegalese forward Sadio Mane struck in the 93rd minute to take Ronald Koeman's side above Manchester United and into third place, ahead of United's trip to West Ham United on Sunday.
Bottom club Leicester City remain three points adrift of safety after losing 1-0 at home to Crystal Palace, who prevailed via a 55th-minute Joe Ledley header.
Meanwhile, Swansea City rescued a point in a 1-1 draw at home to Sunderland after Jermain Defoe's first-half opener was cancelled out by a 66th-minute header from Ki Sung-yueng.
In Saturday's late game, Liverpool visit Everton for what will be Steven Gerrard's last Merseyside derby before he leaves Anfield for the Los Angeles Galaxy.
