|Masters 2018 on the BBC|
|Venue: Alexandra Palace, London Date: 14-21 January|
|Coverage: Watch live on BBC Two, BBC Red Button, Connected TV, the BBC Sport website and mobile app. Live text on the final on the BBC Sport website.|
World number one Mark Selby suffered a first-round defeat by Mark Williams in the opening match of the Masters at London's Alexandra Palace.
Welshman Williams, 42, won a tense deciding frame to clinch a 6-5 victory and reach the quarter-finals.
"I'm over the moon," Williams said. "I'm playing a hell of a lot better than I've done for many years."
In the day's other first-round match, Northern Ireland's Mark Allen beat Belgium's Luca Brecel 6-3.
Williams, who won the Masters in 1998 and 2003, opened with a a break of 135 and moved 2-0 up with an 89 before Selby began to dominate.
The reigning world champion, who admitted the in-form Williams was one of the players he had hoped to avoid in the first round, won three frames in a row en route to a 5-3 lead.
But Williams was not to be denied, winning three frames in succession to avenge his 6-5 loss to Selby at the same stage of this tournament in 2017.
"I thought it was a good match, both of us played quite well," added Williams, who will play Barry Hawkins or Kyren Wilson in the last eight.
"If I can keep playing like that then someone's going to have to play well [to beat me]."
Selby, 34, was also eliminated early in the UK Championship last month, losing to Scott Donaldson in the second round.
In the evening contest, Allen was impressive as he saw off 22-year-old Masters debutant Brecel, compiling breaks of 86, 135 and 72 as the pair shared the first six frames.
Allen, 31, won a scrappy seventh to move 4-3 ahead, before shutting Brecel out in the next two frames and wrapping up the win with a 120 break.
Allen now plays the winner of Tuesday's match between Ronnie O'Sullivan and Marco Fu.
The Masters runs until Sunday, 21 January and the winner will collect £200,000.