As a 14-year-old, Petra Hillenius won her first of two Dutch titles in the 100 m breaststroke. Still a teenager, she qualified for the 1984 Olympics, 1985 European Championships, and 1986 World Championships, although she did not reach any finals. Hillenius then stepped back from the national team, to retire fully in 1988. She was also noted for a self made swimming cap with holes for the swimming goggles - a decade before a similar cap was marketed by Speedo. Hillenius worked as a college teacher, but also remained active in swimming, sitting on the board of the Dutch swimming federation 2014-2018. She retired from these positions when she was diagnosed with ALS, to which she eventually succumbed at age 52.