Keizer totalled 490 official matches for Ajax, scoring 189 goals between 1961 and 1974. He played predominantly on the left-wing and with Ajax won 3 consecutive European Cups (1971, 1972, 1973), having lost the 1969 European Cup final to A.C. Milan. Also with Ajax, he won 6 Eredivisie titles, 5 KNVB Cups, 2 European Super Cups, 1 Intercontinental Cup and 1 Intertoto Cup.