John Higgins admitted it was "a tough one to take" after he crashed out of the BetVictor European Masters in Austria.
The four-time world champion lost three successive frames in suffering a 5-4 defeat against Thailand's Thepchaiya Un-Nooh, who will face Australian Neil Robertson in the quarter-finals.
"It's a tough one to take. I should have won 5-2," Higgins told World Snooker's official website.
"If you don't take your chances the balls don't forgive you. Finishing off matches gets harder as you get older."
Robertson beat Graeme Dott 5-2 while Ali Carter - runner-up in the Masters last weekend - edged past Michael Holt 5-4, but Ding Junhui went down 5-2 against Scott Donaldson.
Another former world champion, Leicestershire's Mark Selby, followed Higgins out of the tournament after a final frame decider against Barry Hawkins.
Selby lost the first four frames as Hawkins hit two century breaks but he then fought back to level the match, helped by a break of 136 in frame seven.
Experienced Hawkins, though, kept his composure to take the match 5-4 with runs of 50 and 59, ending a high-class contest that produced nine breaks of 50 and above.
In other second round games, Gary Wilson beat China's Zhao Xintong 5-1, Marco Fu saw off Xiao Guodong 5-2 and 18-year-old Welshman Jackson Page went down to a 5-3 defeat against Zhou Yuelong.
