Judd Trump and Jack Lisowski have tested positive for coronavirus and been forced to withdraw from the Masters which starts on Sunday in Milton Keynes.
Both players were tested on Friday as part of strict protocols surrounding the event which requires all players to undergo the procedure two days before their opening match.
Trump, who was due to play David Gilbert in his opening match, has been replaced by Joe Perry, whilst Lisowski's spot has been taken by Gary Wilson.
Both Perry and Wilson were present in the tournament 'bubble' as tournament substitutes and have returned negative tests.
World Snooker Tour confirmed the positive tests in a statement, adding: "WST is following extensive Covid-19 guidelines in a 'bubble' environment for this event.
"All players are tested two days in advance of their first match, and after the test they remain in isolation until the result is received.
"All other players and officials tested at the event so far have had negative results.
"Trump and Lisowski will now undergo a further period of self isolation and will receive the support of WST and WPBSA."
Trump beat Lisowski in the final of the last tournament of 2020, the World Grand Prix, shortly before Christmas.
Speaking to the PA news agency on Saturday night, Lisowski clearly had no notion of the imminent positive result, insisting: "I'm on the crest of a wave and I want to keep riding it."
News of the withdrawals is a further blow to the tournament which was forced to give up its hopes of playing at its regular Alexandra Palace venue in front of a limited number of spectators following a tightening of Government lockdown rules.
Open only to the top 16 players in the world, the Masters is considered one of the sport's 'triple crown' events along with the World and UK Championships.
Record seven-time champion Ronnie O'Sullivan is due to begin his campaign against Ding Junhui on Wednesday.
