The Spain striker, who set up Andre Schuerrle's first-half goal, gleefully fired into an empty net after a mix-up between City defender Matija Nastasic and Joe Hart.
Sergio Aguero had levelled soon after halftime for City as victory lifted Chelsea back up to second place, level on 20 points with Liverpool and two behind leaders Arsenal.
Roberto Soldado's 80th minute penalty secured Tottenham Hotspur a 1-0 home win over Hull City and moved them up to fourth, while Sunderland moved off the foot of the table after substitute Fabio Borini lashed in a superb late winner in a 2-1 derby victory over Newcastle United.
Italian Borini, on loan from Liverpool, lifted the gloom at the Stadium of Light when he thundered a 25-metre rocket past a flailing Tim Krul six minutes from time.
Swansea City and West Ham United drew 0-0 in Sunday's other game.
Image: Fernando Torres
Photograph: Clive Rose/Getty Images
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