Ronnie O'Sullivan was stunned by David Grace in the second round of the Northern Ireland Open in Belfast.
The seven-time world champion, who was bidding to win this event for the first time having been runner-up to Judd Trump on three occasions, held an early 2-0 lead over Grace at the Waterfront Hall but apart from an opening break of 62, failed to sparkle like he did at last week's Hong Kong Masters.
The Leeds ace took full advantage and successive breaks of 57, 94 and 64 moved him into a 3-2 lead before O'Sullivan edged a scrappy sixth frame after his opponent missed opportunities to wrap up the match when 40-30 ahead.
Grace, who lost his only other meeting to O'Sullivan 4-0 back in 2018, dominated the final frame as he claimed one of the biggest wins in his career.
The 37-year-old, nicknamed 'Amazing', told Eurosport: "I got over the line in the end, I made hard work of it. You don’t normally get a second chance. When I missed the first chance to win 4-2 that’s when normally the snooker Gods don’t forgive you, but they were on my side today.
“It was going really bad at 2-0 and I’m proud that I managed to find the cue-ball control to make a couple of breaks in the middle of the match to give myself a foothold.
“When you play the legends they get better when they go in front, they don’t normally give you a foothold so it was nice to be able to give myself a chance.”
Grace is a painter in his spare time and added: "I did a painting of Ronnie last summer but I don't have much time to do it anymore with all the snooker. I'm in the club every day trying to prove I can climb the rankings and do it on the biggest stages."
The world number 49 also expressed his gratitude at World Snooker's decision to guarantee all the players a minimum earning of £20,000 this season, saying: "It's absolutely massive. I looked at my first five years as a pro and I was losing money every season.
"I was fortunate I had the support network to get through the lean years but many don't have that."
Grace will play Tian Pengfei in round three after Tian made a break of 55 in the decider to beat Martin Gould 4-3.