Rengeruk had her first successes in 2013, winning the Kenyan youth trials race before beating Ethiopia's Berhan Demiesa to take the 3000 metres title at the 2013 World Youth Championships in Athletics. The following year she placed fifth in the junior race at the African Cross Country Championships, sharing in the team gold. She pre-fixed a win at the Kenyan junior trials with a 3000 m best of 8:53.41 minutes on her IAAF Diamond League debut in Doha. At the 2014 World Junior Championships in Athletics, however, she was outdone by American home athlete Mary Cain, leaving Rengeruk with the silver medal.