The 28-year-old, who overcame his opponent 10-4, battled through illness and blurred vision to book his place in the next round.
"I've got a few days to rest now," Allen is quoted as saying by the Daily Mail. "I've had a bit of food poisoning, and haven't been feeling too good the last few days.
"An hour and a half before the match I was in the NHS walk-in centre, trying to get treatment. At times out there I couldn't see the balls, I had blurred vision. I wasn't able to see anything more than three feet away, it was a guess.
"Thankfully I managed to guess right on a few. I would never pull out, no way, it's the World Championship. They'd have to scrape me off the table, no matter how bad I feel."