The World Snooker Championship welcomed an extraordinary guest at Sheffield's Crucible theatre when a pigeon swooped down and walked across the table.

The feathered guest briefly alighted on the baize in the afternoon session of the match between defending champion Mark Selby and Yan Bingtao on Friday.

"Well, I don't believe that," BBC commentator and former world champion Ken Doherty said.

"We've got a pigeon in the Crucible," he added with a giggle, in case the identify of the winged intruder remained a mystery.

Yan appeared briefly perturbed as the pigeon flew past him to the floor, before hopping up onto the table, where the referee was racking the balls for the sixth frame.

It then startled Selby, who was just returning from backstage through the players' entrance to the playing arena.

"Oh, it's coming up to the commentary box," Doherty said.

"Hello there," veteran commentator John Virgo said.

"Where's the pigeon going?" he added.

Back outside to the relative safety of Sheffield, it turns out, after staff managed to capture the intruder and release it.

The pigeon provided a brief footnote in what was an enthralling encounter between the two players, which was won by 22-year-old Yan, 13-10.

The match featured the longest individual frame in World Championship history, with the 22nd lasting a whopping 85 minutes and 22 seconds.

Yan is the latest in a line of Chinese players to launch an assault on snooker's holy grail.

Selby, who revealed pre-tournament that he had struggled with his mental health and had been unsure as to whether he would defend his title, told the BBC Yan had the ability to go all the way.

"I wish I was as good at his age," Selby said. 

"For me, he has the game to go on and win it."

Ding Junhui remains the only Chinese player to reach a World Championship final, when he lost to four-time champion Selby in 2016.