Chelsea slipped up in their bid to overtake Liverpool at the top of the Premier League as they were held to a 1-1 home draw against West Ham.The Blues, who welcomed Gianfranco Zola back to Stamford Bridge for the first time since he played for the club, went behind when Craig Bellamy smashed a low shot past Petr Cech after good work from Mark Noble.But the introduction of Didier Drogba sparked Chelsea into life in the second half, as he fed Frank Lampard who then lifted the ball over for Nicolas Anelka to stab home the equaliser in the 51st minute - his 100th Premier League goal.The Blues had chances to win the game, with Rob Green pulling off a great save from Drogba; but it was Carlton Cole who missed the best chance to win the game for the Hammers when he was through one-on-one with Petr Cech and saw his shot saved.