Tiger Woods says he can not comprehend the emotional pressure Phil Mickelson will be under when the US Open gets under way on Thursday.
Mickelson delayed his arrival at the Bethpage Black course to spend his 39th birthday with wife Amy, 37, who was diagnosed with breast cancer in May.
"I couldn't imagine dealing with what he has to deal with on a daily basis," said Woods.
"And hats off to how he's handled it because certainly it's so hard to do."
Bethpage Black was the course where Woods edged out Mickelson by three strokes to claim the title in 2002, but the runner-up won loud and enthusiastic backing from the Long Island crowd.
The pair have been reported as having a frosty relationship, but 14-time major winner Woods revealed that he and his wife have spent time with the Mickelsons socially. "All the years that we've played doubles and table tennis, Elin and myself and Phil and Amy, those are priceless times," he said.
"Myself and everyone out here hopes that she gets well and she's back out here as soon as she can."
Woods' father Earl died in 2006 after an eight-year battle with prostate cancer and the world number one said that tournament golf offered him little distraction during the illness.
"You're surrounded by people wishing you well the entire time and hoping everything works out. But then again, they keep reminding you of the same circumstance you're dealing with on a daily basis," he said.
Mickelson withdrew from the PGA Tour in May to spend more time with Amy whose doctors have delayed her treatment until July, believing they have caught the disease in its early stages.
With the US Open believed to fit around his wife's treatment, Mickelson returned to action, finishing 59th at the St Jude Classic in Memphis last week.
He has a record four second-place finishes in the US Open.
Northern Ireland's Darren Clarke, who lost his wife Heather to breast cancer in 2006, said he had spoken with the American.
"We had a good conversation about various things. He's a good friend. It's very sad news but it's early stages so we all hope the tests and everything else go as well as we wish," he said.