Northern Ireland's Mark Allen won five frames in a row as he came from behind to defeat Wales' Mark Williams 6-2 in the first round of the Masters - despite suffering from illness before the tournament.

In a match between two former champions, Allen took the opener thanks to a break of 67, before 50-year-old Williams battled to take the next two to lead 2-1.

Allen, 39, made a break of 54 in frame four to ensure it was 2-2 at the mid-session interval and then raised his game with breaks of 60, 125 and 70 to take control of the match.

Speaking to the BBC after the match, Allen revealed his preparations had been hampered.

"I felt comfortable, which is weird," said Allen. "I prepare properly for these events but I couldn't prepare for this at all as I've been lying in bed all week with food poisoning, so I just thought 'go out there and give my best'.

"I wouldn't have had much left if it had got much closer."

It was a frustrating afternoon for Williams as mistakes crept into his game. He was unable to register a single break of 50 and had a pot success rate of only 71%.

His departure means three former world champions have been beaten in the opening three matches after Shaun Murphy and Mark Selby lost against Wu Yize and Xiao Guodong respectively on Sunday's opening day.

In the quarter-finals on Thursday, Allen will face either world number one Judd Trump or China's Ding Junhui, with those two meeting in their last-16 tie on Wednesday.

Allen beat Kyren Wilson to win the 2018 Masters and said he "was not getting carried away" about his hopes for a repeat success.

"This is my 11th quarter-final [at the Masters] but I've only won it once so I have to sort that out - this is as good a week as any to do that," added Allen.

Williams thought he "made it easy" for his opponent and said: "I was pretty poor. It can happen and will happen more. I tried, even though it didn't look like it."