Answer
Roger Federer was defeated by John Millman in the fourth round of the 2018 US Open, marking a significant upset in the tournament. Millman, an Australian player ranked 55th in the world at the time, displayed remarkable skill and determination throughout the match, ultimately prevailing over the 20-time Grand Slam champion in straight sets, with the final score being 3-6, 7-5, 7-6 (7), 7-6 (3).