Roger Federer of Switzerland hits a forehand against Jose Acasuso of Argentina during day three of the Western & Southern Financial Group Masters on August 19, 2009 at the Lindner Family Tennis Center in Cincinnati, Ohio. (Nick Laham/Getty Images)
Ferrer held a 3-1 lead in the deciding set but Federer broke back immediately and then again in the ninth game before serving out for a place in the last eight.
World number 19 Ferrer had won just one set in eight previous meetings with Federer but the Swiss was all over the place early on before eventually finding his rhythm.
Federer will go on to play a quarter-final against either Lleyton Hewitt or Sam Querrey.
