Chelsea moved to the top of the Premier League with a hard-fought victory over a stubborn Middlesbrough side on Sunday.
Victor Valdes produced a sublime first-half save to keep the scores level, showing great reflexes to turn Pedro's right-footed shot over the bar but the Boro goalkeeper was eventually beaten in the opening period when Costa poked home.
The Brazil-born Spain international was alert to volley home from close range after Middlesbrough failed to clear their lines from a corner.
Chelsea continued to search for a second after the the break and were unfortunate not to do so when David Luiz and Costa combined to set-up Pedro but the Spain winger's effort cannoned off the crossbar.