Day was three down after six holes of his opening match against Pat Perez when he conceded and walked off the course, prompting speculation that he had suffered another injury.
However, the 29-year-old Australian then told a hastily-arranged press conference that he had found it impossible to focus on golf due to his mother Dening’s battle with lung cancer.
“My back is fine, I’m 100 per cent healthy,” Day said. “My mum’s been here (the United States) for a while and she has lung cancer. At the start of the year she was diagnosed with 12 months to live.
“The diagnosis is much better being over here, she’s going into surgery this Friday and it’s really hard to even comprehend being on a golf course right now because of what’s she going through.
“She had all the tests done in Australia and the docs said she was terminal and she only had 12 months to live and I’m glad I brought her over here because of it.
“It’s been really hard to play golf lately this year. It’s been very, very emotional as you can tell.”
Day, who was 12 years old when his father died from stomach cancer, added: “I’ve already gone through it once with my dad and I know how it feels and it’s hard enough to see another one go through it as well. As of now I’m going to try and be back there with my mum for surgery and make sure everything goes right with her.
“Emotionally it’s been wearing on me for a while and I know my mum says not to let it get to me but it really has, so I just need some time away with her to make sure that everything goes well because this has been very, very tough for me.
“I’m going to do my best and try and be there the best I can for her because she is the reason that I’m playing golf today.
“I’ve obviously pulled out this week because of my mum going into surgery to try and get rid of this three or four centimetre mass that’s in her lungs. I’m just hoping for a speedy recovery for her and we can get this behind us and she can live a long life.”