That pair lead by two over American Cameron Wilson from the Shorehaven Gof Club in Connecticut.
Gold Coast based Tasmanian Cory Crawford is one shot further back after his round of 71.
The round of the day came from Brisbane's Aaron Wilkin, his 66 allowing him to recover from a very slow start to the event to move within five shots of the lead.
Sinnott's resume includes a victory at the 2011 Dunes Medal but he has been in good form in more recent times also with a runner-up finish at the Victorian Amateur Championship a month ago.
Viitakangas, from the Palmer Gold Coast Golf Course in Robina, has had a share of or the outright lead all week and faces a big day tomorrow to maintain the intensity needed to lead from go to whoa.