Ali Carter made the most of his unexpected opportunity to play at the Dafabet Masters by beating three-time champion Mark Selby in his opening match.
Carter was only invited to feature at Alexandra Palace after seven-time winner Ronnie O'Sullivan pulled out of the tournament in December.
But the world number 17, who has lost at this stage in all but two of his previous 11 appearances, booked his place in the quarter-finals with a 6-4 victory over Selby.
After starting brightly, Carter saw a 3-1 lead wiped out when Selby won three frames on the spin, making a break of 94 along the way.
However, Carter recovered to win the next three with breaks of 82, 63 and 68 and will play John Higgins or Barry Hawkins in the last eight.
"I was saying (beforehand), you shouldn't be there and it's a free shot, but then your competitive instinct comes out and you can't help but think that this is a chance, don't waste it," Carter told Eurosport.
Ding Junhui was also on the receiving end of an upset as he lost to Joe Perry earlier in the day.
World number 16 Perry knocked out Ding with a 6-3 win, as breaks of 93 and 83 helped him to three successive frames after the match was tied at 3-3.
The Chinese player, who was unable to build on a sublime 135 visit in the second frame, was the UK Championship winner last month but was not at his best.
"If you draw a top player like that and they are on their A-game you are up against it, but if they are missing the odd one or two you have to be there and take your chance," Perry told the World Snooker Tour's official website.
Ding added: "The match was a 50-50 one, but I didn't take enough chances.
"I was losing position of the cue ball, trying to pot hard shots and missing every time."