Chelsea missed the opportunity to go eight points clear at the top of the Premier League by drawing 1-1 against Southampton in an entertaining game at Stamford Bridge where the officials' decisions will once again be scrutinised.
Jose Mourinho's men were looking to respond from their European exit by tightening their grip on the title after Manchester City's loss at Burnley and went in front through Diego Costa's first Premier League goal in almost two months.
Dusan Tadic equalised after a contentious penalty, awarded for a foul by Nemanja Matic on the impressive Sadio Mane.
Replays showed Matic slid in from behind and made contact with the ball, but referee Mike Dean pointed to the spot and booked the Chelsea midfielder.
Chelsea might have had a spot kick of their own for a Tadic foul on Branislav Ivanovic, but then Dean gave the Blues the benefit of the doubt rather than reduce them to 10 men for all but the opening minute of the second half.
Matic could have marked his domestic return following a two-match ban for his dismissal against Burnley with another sending off, but he avoided a second booking for a foul on Mane within seconds of the restart.
Chelsea, under pressure for much of the contest, finished on the attack, but Fraser Forster made fine saves from Oscar, Eden Hazard and Juan Cuadrado to earn Saints a point.
The Blues, who are six points clear at the top, still have a game in hand on City and remain firmly in control of their bid for a first Premier League title in five years.
