In 2003, Kupets won the U.S. Classic and then became the national champion for the first time. She was named to the U.S. team for the 2003 World Championships in Anaheim, California, and competed well in the qualifying round, contributing to the USA's high placement and qualifying to the all-around and uneven bars finals. However, during a practice session before the team finals, she tore her left Achilles tendon on a double Arabian somersault and was forced to withdraw from the event, returning home to Maryland immediately for surgery. The U.S. team went on to win the gold medal. Kupets was still considered a member of the squad and received credit and a medal for her contributions in the preliminaries.