Antonio Conte side Chelsea maintained a seven-point advantage over Tottenham Hotspur at the top of the English Premier League with a hard-fought win over Manchester City.
Pep Guardiola side suffered league defeats home and away to the same competitor in a single league season for the first time in his managerial career as the visitors are left to fight for a top-four place.
Belgium mid-fielder Eden Hazard gave the Blue side a 10th-minute lead when his shot deflected off returning City skipper Vincent Kompany past goalkeeper Willy Caballero, who should have done better.
The host goalkeeper Thibaut Courtois was also badly at fault when his poor clearance to midfielder David Silva set up Sergio Aguero's equaliser after 26 minutes - but the group leaders were back in front before half-time.
They were awarded a penalty after midfielder Fernandinho tripped Spanish winger Pedro and even though Willy Cabellero saved Hazard's penalty-kick, the rebound fell kindly for the former Lille player to score.
Manchester City, who stay in fourth position, had the better chances in a tense second period, with defender Kompany's header bouncing back off the bar and centre-back John Stones shooting over from six yards in injury time. Chelsea, however, held on for a victory that was even more vital given Tottenham Hotspur dramatic late comeback at Swansea City.