The quarter-finals were played on 18 and 19 January 2018. Defending champion Ronnie O'Sullivan was knocked out of the tournament by Mark Allen, who won their quarter-final match 6–1. This was O'Sullivan's first loss at the Masters since 2015. He claimed afterwards that he had been feeling ill before the match and was "glad" to be out of the competition. He also revealed, however, that he had not been feeling well during his whitewash of Marco Fu in the first round. After the match, Allen commented on the negative crowd reaction, saying that his playing style had been affected by some of O'Sullivan's fans.