Mark Williams insists he "could not care less" about the prospect of reclaiming the world number one spot following Mark Selby's stunning defeat to unranked amateur James Cahill at the World Snooker Championships.

Cahill's 6-3 success, which he wrapped up with a stunning long pink just past midnight on Friday morning, left Selby's seven-year hold on the year-ending top spot looking increasingly precarious.

World champion Williams, who opened his account in York with a 6-2 win over Adam Duffy, will return to the number one position for the first time since 2011 if he goes on to win the tournament.

But Williams, who has made no excuses for his relaxed attitude to the sport since his remarkable win in Sheffield in April, says he has no desire to once again become snooker's leading light.

Referring to Ronnie O'Sullivan, Williams said: "It's something I'm not interested in whatsoever. We all know who is the best player in the world and he just doesn't enter as many tournaments.

"To be honest, I don't even want it. I've been practising so little I don't even know if it's still the same carpet in my club. If I don't win another tournament I'm happy – it's still going to be a party."

For Cahill, career goals remain much more important as he prepares for his second-round match on Saturday against his occasional practice partner, Sunny Akani.

Despite being only 22, Cahill has already endured a rollercoaster career, beating Ding Junhui on his way to the fourth round of the same tournament as an 18-year-old in 2014, before a form lapse so severe it left him questioning his future.

Cahill only got into this year's Championships due to withdrawals among the 127 world-ranked players, and is reliant on help from parents and sponsors as he battles to regain the tour card he lost last year.

Cahill said: "I used 11 different cues in my last season on the Tour and I was all over the place, so I needed a break. I'm convinced I'm good enough to compete with the best and I need to start showing that."

Cahill, a former nephew of Stephen Hendry, who was once married to his aunt, added: "It's hard to play full-time when you don't have a job. But I've got some good people behind me now and I can start looking to the future."

Selby was the second big name to crash out before the televised stages, with another former world champion, Shaun Murphy, having exited the tournament on Wednesday.

With his top ranking now in jeopardy, Selby said: "One of my aims is to stay there as long as possible. But it's going to happen at some stage, and if it's the end of this season, so be it."