He may be a former US Open champion, but an important change to this year's tournament format had escaped Jordan Spieth's attention.

The USGA announced in February that it had abandoned its 18-hole, next-day format for play-offs in favour of a two-hole aggregate play-off on Sunday evening, starting with immediate effect at Shinnecock Hills.

But that was news to three-time major winner and 2015 champion Spieth when he was asked about the change at his pre-tournament press conference.

"It's the first I've heard of that being an option," said Spieth, who would have faced an 18-hole play-off with Dustin Johnson at Chambers Bay had the world number one not missed from three feet for birdie on the 72nd hole.

"I didn't even know. I guess strategy changes a little from an entire round, but I honestly had no idea that it even changed.

"I was even looking at a weather forecast for Monday, thinking what's it look like if you happen to work your way into a play-off. Shows you what I know."

Spieth is the only member of the world's top six without a victory this season, with his putting coming under intense scrutiny, but the 24-year-old is confident he can enjoy a strong second half to the season.

"I think my patience has been tested, just not going into Saturday or Sunday with a legitimate chance to win," added the former world number one, who needs to win the US PGA Championship to complete a career grand slam.

"Compared to previous years, the limited number of chances I've had on the weekends has been frustrating. I feel like my game is in the best shape it's been in a long time, including last year.

"And my results don't necessarily speak towards that, but I feel that way, and so I'll stick with the process, and they'll surely come at some point."