Olympic champion Katherine Grainger won her first race at the Great Britain senior trials since 2012 by beating event favourite Victoria Thornley in their women's single scull heat.
The 39-year-old Scot, who took two years out from rowing after winning double sculls gold at London 2012, goes into Sunday's semi-final.
Fellow Olympic champion Kat Copeland won her lightweight single scull heat.
Helen Glover and Heather Stanning, also winners in 2012, won the women's pair.
Glover and Stanning will next race in Sunday's semi-finals, as will Copeland.
All the rowers are competing for a place in May's European Championships in Poland. The squad will be announced on 13 May.