Defending champion Jason Day broke down in tears after withdrawing from the WGC-Dell Technologies Match Play to spend time with his mother as she undergoes treatment for lung cancer.

Day was three down after six holes of his opening match against Pat Perez when he conceded and walked off the course in Austin, 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."

The circumstances behind Day's withdrawal understandably overshadowed results from earlier in the day, which saw Rory McIlroy, Jordan Spieth and Danny Willett all beaten.

McIlroy looked to be on course for victory against Soren Kjeldsen when he overturned a two-hole deficit with three birdies in a row from the 11th, only for the 41-year-old Dane to birdie the next four holes to win 2&1.

Two-time major winner and Texas native Jordan Spieth won just one hole before losing 4&2 to Japan's Hideto Tanihara, while Masters champion Willett's recent struggles continued as he lost to Korea's KT Kim by the same score.

Dustin Johnson was the only player to reach the knockout stages last year after losing his opening match, but the world number one had no such problems this year after thrashing Webb Simpson 5&3.

Ross Fisher, Matt Fitzpatrick and Andy Sullivan suffered heavy defeats to Louis Oosthuizen, Kevin Na and Branden Grace respectively, but there were victories for Tommy Fleetwood over Zach Johnson, Paul Casey against Joost Luiten and Tyrrell Hatton over Charles Howell.

Lee Westwood lost 3&2 to Arnold Palmer Invitational winner Marc Leishman, while Scotland's Russell Knox was a 3&2 winner over American Bill Haas.

In the day's final two matches, Spain's Jon Rahm defeated Kevin Chappell 3&2 before compatriot Sergio Garcia salvaged half a point against Ireland's Shane Lowry.

Garcia was two up after five holes but lost four of the next five before stopping the rot by winning the 12th with a par five. Lowry maintained his lead until the final hole, but found trouble off the tee and three-putted from long range after Garcia failed with his birdie attempt.