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The world No. 3 walked off the course after playing six holes of his match with Pat Perez in Austin, Texas before giving an emotional press conference.
Day explained his mother had been diagnosed with lung cancer at the start of the year in Australia and given 12 months to live.
She has since joined the 29-year-old and his family at their home in Ohio, where she has had further tests.
"The diagnosis is much better being over here," the 2015 US PGA champion told reporters.
'Very emotional'
"She's going in for surgery this Friday, and it's really hard to even comprehend being on the golf course right now because of what she's gone through. I'm glad I brought her over here."
Dening now faces surgery at the James Cancer Center at Ohio State University on Friday to remove a 3-4 cm mass on her lungs.
Day was 12 when his father Alvyn died of stomach cancer. He subsequently began drinking, fighting and getting in trouble regularly.
Dening took out a second mortgage on their house and borrowed money to send Day to a boarding school famed for its golf academy. It was there his association with caddie and coach Col Swatton began.
"It's been really hard to play golf lately," Day explained. "It's been very, very emotional, as you can tell. I've already gone through it once with my dad. And I know how it feels.
"As of now, I'm going to try to be back there with my mom for surgery and make sure everything goes right with her.
"Emotionally, it's been wearing on me for a while and I know my mom 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. Family is first. It's just a hard time.
"I'm hoping for a speedy recovery for her, and we can get this behind us and she can live a long life."
'Gutsy guy'
Day was defending champion at the WGC Match Play after winning the the title for the second time in 2016, beating South Africa's Louis Oosthuizen in the final.
After a string of second-place finishes, Day finally got his hands on his first major title in 2015 at the US PGA Championship.
Day, who later that year became world No. 1 for the first time, said after his three-stroke triumph at Whistling Straits: "Without my mom's sacrifice, there is no doubt I would not have made it.
"Her and my sisters did so much for me so I could have the chance to succeed and I will forever be grateful. There is no telling where I would have ended up. Maybe jail. Maybe dead. But she made sure that wasn't my path."
The first major of 2017 is two weeks away, but it is unclear at this stage whether Day will take his place at Augusta National for the Masters.
"Whatever happens, happens. Whether he's playing that ... I know it is something he wants to do," his manager Bud Martin was quoted as saying by the PGA Tour website.
"But anyone that has a mother, I think you know that she comes first. Being by her side in Ohio when she has the surgery is something that I think just overwhelmed him. He thought that was where he needed to be.
"Jason is a gutsy guy. You don't accomplish what he's accomplished on the golf course without having resilience."