Wyndham Clark's late mother always told him to "play big" and the American did just that at the U.S. Open to earn an emotional breakthrough victory on Sunday.

Having seen his three-shot lead over Rory McIlroy whittled down to one, and with two daunting par fours still ahead of him at the Los Angeles Country Club, Clark drew inspiration from his mother Lise, who died of breast cancer in 2013 at the age of 54.

After his winning putt dropped on 18, Clark's thoughts immediately turned to her.

"My mom was so positive and such a motivator in what she did," Clark said. "She called me 'Winner' when I was little, so she would just say, 'I love you, Winner.' She had a mantra of 'play big'."

The 29-year-old from Denver said his mother used to leave notes of encouragement in his golf bag, gym bag and backpack when he was young, sometimes leading to embarrassing moments in front of friends.

Those notes turned to text messages as he got older and he was away playing tournaments.

He said he misses those exchanges now.

"I was a mama's boy, so there would be a lot of hugging and crying together," he said.

"But I know she'd be very proud of me. I miss her, and it's obviously great to think about her, and being here in LA and winning something like this makes me think of her even more than maybe my day-to-day when I'm not playing a championship."

Clark broke down in tears as soon as his short par putt sealed the win and covered his face with his hat before his playing partner Rickie Fowler embraced him.

"Your mom was with you," Fowler told Clark.

"She'd be very proud."