Kammala wannan jumla: A tsakiyar aikinsa na shekaru hudu tare da TPS a cikin ƙananan SM-liiga, Flyers sun zaɓi Niittymäki a matsayin na 168 a cikin duka
1998 NHL shigar da daftarin aiki. Ya ci gaba da wasa a Finland tare da TPS a shekararsa ta farko daga wasannin matasa, kuma ya lashe kofin Rookie of the Year na gasar zakarun Finland. Bayan karin yanayi biyu tare da TPS, Niittymäki ya koma Arewacin Amurka don shiga Philadelphia Phantoms a cikin American Hockey League (AHL). Lokacin da Phantoms suka lashe gasar zakarun AHL Calder Cup, an kira shi MVP na wasan kwaikwayo. Daga nan ya buga wasanni hudu na Philadelphia Flyers kafin ya sanya hannu a matsayin dan wasa mai kyauta tare da Tampa Bay Lightning. Bayan shekara guda, ya sanya hannu a matsayin dan wasa mai kyauta tare da San Jose Sharks, amma bai buga wasanni da yawa a gare su ba a cikin shekaru biyu masu zuwa saboda raunin hip. Ya buga kakar wasa ta karshe tare da kungiyar matasa ta TPS, kuma ya sanar da yin ritaya a shekarar 2013.