Graeme Dott became the first champion since Stephen Hendry in 2000 to suffer an opening-round exit after a 10-7 defeat to Preston's Ian McCulloch.
Dott was second best all match
Dott looked tense all match and will have rued the missed pink that would have given him a 5-2 lead.
McCulloch ground down the Glaswegian although he showed he is adept at heavy scoring too with a 15th frame 110.
Dott claimed the 14th with an 85 and then reduced the deficit to 9-7 before McCulloch took a tense 17th.
The 29-year-old is in illustrious company, with John Spencer, Terry Griffiths, Steve Davis and Dennis Taylor also having exited at the first-round stage as champions.
Dott partly blamed Ian McCulloch's style of play for his first-round exit.
"The way Ian plays frustrated the life out of me, but I beat myself really," said champion Dott.
"I've never felt like that. I felt as though I was letting him beat me. I felt absolutely horrible, that defeat has ruined a perfectly good season.
"I didn't want to say anything before the event in case it gave Ian a boost."
McCulloch breathed a sigh of relief after clinching the victory.
He said: "I like Graeme, I don't get any satisfaction about beating him.
"He didn't fancy beating me. He made it hard, I made it hard, but it felt really good going into the last session and felt like I could beat him.
"You don't become world champion if you're a muppet. Graeme has been a good world champion, but I'll take any win."
Graeme Dott 7-10 Ian McCulloch
43-71 64-37 71-17 (61) 1-63 74-41 68-37 61-65 32-85 (51) 57-62 46-58 59-28 31-67 20-68 101-8 (85) 17-110 (110) 85-20 24-71