Paul Dunne and Graeme McDowell are among the first out at the US Open in Wisconsin today.
Dunne starts after noon while former winner McDowell begins just after 1pm. Shane Lowry and Rory McIlroy start their rounds this evening.
Play may be delayed over the next four days as bad weather is forecast.
Dustin Johnson insists disrupted preparations have not left him "behind the 8-ball" as he seeks to defy the odds and make a successful title defence.
Curtis Strange was the last player to win back-to-back US Open titles, following his play-off victory over Nick Faldo in 1988 with a one-shot win 12 months later.
And since 1991, only five champions have finished better than 15th in defence of the title, while eight champions missed the cut in the same period.
But despite only arriving at Erin Hills on Tuesday afternoon after Paulina Gretzky gave birth to the couple's second son - named River Jones - on Monday, Johnson was in a typically relaxed mood in his pre-tournament press conference.
"It's been very fun the last couple of days, obviously having a new son on Monday was awesome," Johnson said. "Everybody's healthy so that's good.
"It actually worked out pretty well that I missed the cut at Memorial, even though I didn't want to. I came up here and got to practice for two days. It definitely helped with my preparation coming in because I knew I wasn't going to get here until Tuesday afternoon.
"Even though I came in late I didn't feel like I was behind the 8-ball at all because I got to prepare 10 days ago or so.
"It helps that Paulina and my son are at home now. She's a lot more comfortable and everybody is healthy. Obviously I don't have to worry about them and that's a big relief.
"But now I've got to play golf. This is why I'm here. I'm here to play golf. I'm here to compete. So that's what my job is this week."
Johnson has gone from strength to strength since his victory at Oakmont 12 months ago, which was achieved despite not knowing for the last seven holes of the final round whether he would be penalised a shot for an earlier rules infraction.
The world number one won three tournaments in succession earlier this year - including back-to-back World Golf Championship events - before a back injury forced him to withdraw from the Masters.
"I'm excited to come back and defend this week," he added. "The game feels like it's in really good shape. I've been working really hard at it the last couple of days.
"It feels good to still be number one. I'm here to compete. I'm here to give myself a chance to win on Sunday. But I'm going to have to play really well if I want that chance."