Shaun Murphy believes snooker has to take the rough with the smooth where Ronnie O'Sullivan is concerned.

Four-time winner O'Sullivan returns to the sport on Saturday as the Betfred World Championship gets under way at the Crucible, with an opening match against Scotland's Marcus Campbell.

The 37-year-old will be defending his world title, but after almost a year away from snooker on a career hiatus there can be no guarantees as to how he will perform.

Knowing that snooker's greatest talent could be rusty and perhaps vulnerable in the early stages adds spice to the tournament, and O'Sullivan has admitted there is no telling how he will get on.

Murphy, world champion in 2005, makes Neil Robertson and world number one Mark Selby his picks for the title, but he believes O'Sullivan will be the player to fear if he can get into an early groove in Sheffield.

When asked if O'Sullivan's attitude towards snooker showed a lack of respect, Murphy said: "I think he has had his moments, good and bad, with snooker. But they have been well publicised.

"He has been his own worst enemy through the years, but somebody that naturally gifted is a genius, and they are always just slightly off the beaten track.

"When he is in full flow he is unbeatable, and when he's not, he is still hard to beat. I would make Mark Selby and Neil Robertson favourites for the tournament, on form, rankings and all the rest of it."

"I'm pounding the streets where I live and really putting some miles in" - Shaun Murphy

"Selby is a proven winner of the majors now - he has not won the World Championship, but he has won the others. Robertson is a previous world champion.

"I think it is a very tough one to predict this year, but I do think it will be one of the main players that comes through the field and wins."

Murphy, 30, starts against qualifier Martin Gould on Saturday afternoon, after O'Sullivan and Campbell's morning session.

The 17-day tournament is often dubbed a marathon and Murphy has been sharpening up his fitness by preparing for a charity run.

Sale-based Murphy is a long-time supporter of the Royal Manchester Children's Hospital Charity and will be raising money by running in the Great Manchester Run, a 10-kilometre road race, on May 26.

"I would definitely rather do 10k around a snooker table!" Murphy said.

"I'm not a natural-born athlete - I was never into that sort of thing as a kid, so it's quite a big commitment for me to do the Manchester 10k.

"Training is going well for it at the moment. I'm pounding the streets where I live and really putting some miles in. So I'm getting there - I'm not quite there yet, but hopefully come the end of May I'll be out there and maybe I can cross the line with a flourish."