Chelsea threw the Premier League title race wide open by losing 1-0 at Aston Villa on Saturday, ensuring Manchester City trimmed the gap to the leaders to six points with a battling 2-0 win at Hull.
Chelsea saw Willian and Ramires sent off, and manager Jose Mourinho sent to the stands, in a wild end to a match settled by Fabian Delph's impudent 82nd-minute goal at Villa Park.
City leapfrogged Arsenal and Liverpool into second place by overcoming the 10th-minute sending-off of captain Vincent Kompany to win at Hull thanks to goals by David Silva and Edin Dzeko.
City has three games in hand over Chelsea, while Liverpool and Arsenal are seven points back having played two fewer matches.
The battle to avoid relegation also heated up, with the bottom seven teams separated by just five points after last-place Fulham beat Newcastle 1-0 for the team's first victory under new manager Felix Magath.
Cardiff conceded a goal three minutes into injury time to lose 2-1 at Everton and is now just a point above Fulham along with Sunderland, which drew 0-0 with Crystal Palace.
West Bromwich Albion won 2-1 at Swansea to move three points clear of danger, Norwich remained in trouble by losing 4-2 at Southampton and Stoke beat West Ham 3-1 in a match between two mid-table teams.
Chelsea looked to be marching to the title in Mourinho's first season back at Stamford Bridge, but a first defeat in 15 league matches offers plenty of encouragement to its title rivals. Arsenal and Liverpool will look to take full advantage on Sunday when they play Tottenham and Manchester United, respectively.
Willian had already been sent off for two bookings, the second coming in the 68th for a slight tug of an opponent's jersey, when Delph ran onto Marc Albrighton's cut-back and flicked a left-footed finish from behind his right foot and into the corner.
Ramires was then dismissed for a lunging tackle on Karim El Ahmadi that was described by Villa manager Paul Lambert as a "leg-breaker." In the ensuing melee, Mourinho was ordered from the dugout by referee Chris Foy.
"I asked for five seconds to ask one single question," Mourinho said. "I had no question because I was sent to the stands."
Doubts were suddenly swirling around Man City's title bid after a slump in form that saw the team lose to second-tier Wigan in the FA Cup quarterfinals, eliminated from the Champions League by Barcelona, and slip nine points off the pace in the Premier League.