Already without a father from an early age, the Australian major-winner's form has suffered since news earlier this year she might have had as little as 12 months to live.
A 3-4 cm mass was successfully removed from her left lung in March.
"My mum getting lung cancer was very difficult," Day told reporters Tuesday ahead of this week's CJ Cup in South Korea.
"I don't have any grandparents. I don't have my dad. I don't have anyone. If I lost my mum all I would have would be my two sisters."
Ahead of the surgery, Day withdrew from March's WGC Match Play event
against Pat Perez after just six holes, breaking down in tears.
Dening had made the young golfer's dream possible, steering him away from trouble in his teenage years and taking out a second mortgage on their house in Queensland to fund his place at a prestigious school.
Without her, Day stresses, there is no doubt he would not have made it to the top.
"Realizing how much she sacrificed for me growing up, it meant a lot to me to be there for her," said Day, who invited his mother to stay with him at his family home in Ohio to facilitate her recovery.
"I didn't want to be on the golf course. I knew that if she was going to die then I wanted to spend as much time as I could with her, because I only get to see her once a year.
"Fortunately the surgery went well and now I'm able to focus back on golf."
After a series of second-place finishes, Day finally secured his major tournament breakthrough at the PGA Championship in 2015
The 29-year-old held the world No. 1 ranking for a total of 51 weeks before being replaced by 2016 US Open champion Dustin Johnson.
But despite emerging victorious at eights events on the PGA Tour in the two years prior to his mother's illness, he has been unable to win one since.
"It was a little bit of a combination of things why I had this poor season. I was a little bit burnt out from 2016 and 2015," said Day.
"I didn't know how to handle being number one. Next time I get there again I will handle it a little bit differently.
"I still worked very hard to get through that burnt-out stage. And then my mum got lung cancer.
"Hopefully the distractions that I've had on the golf course and off the golf course have gone away and I can focus on climbing back to number one."