Novak Djokovic retires from his Australian Open semifinal after losing the first set against Alexander Zverev

Alessandra Tarantino/The Associated Press

Ukrainian skeleton athlete Vladyslav Heraskevych, a likely medal contender at the Milan Cortina Games, was barred from racing Thursday after refusing a last-minute plea from the International Olympic Committee to not use a helmet that honors more than 20 athletes and coaches killed since Russia invaded his country four years ago.