Friis was born in Blovstrød. Finishing third in the 1500 metre freestyle competition at the European Championships 2008 in Eindhoven she won her first major long course medal. At the 2008 Summer Olympics in Beijing, she won bronze in the 800 metre freestyle competition. At the 2009 World Championships she won silver in the 1500 metre freestyle competition and gold in the 800 metre freestyle competition with the second fastest time ever and a new championship record. At the 2011 World Aquatics Championships in Shanghai Friis won gold in the women's 1500 metre freestyle in the time 15:49.59 and silver in the women's 800 metre freestyle. Friis participated in the women's 400 metre freestyle and reached the final. She finished in fifth in a time of 4:04.68, which was a new Nordic record.