Matsuyama, the 2021 Masters champion, opened with an eight-under 63 on Thursday to sit two shots clear atop a star-studded leaderboard.

Red-hot Xander Schauffele, the defending Olympic gold medallist who has won his first two major titles this year at the U.S PGA and Open Championship, was in outright second at 65.

Day was the best of the Australians after signing for a two-under 69 to be tied for 21st.

And it would have been much better if not for a double-bogey on the 18th hole, an abberration he blamed on “two uncommitted swings”.

“This is probably the most nervous I’ve been wearing a set of clothes – you look down and see the (green and gold) colours,” said the former world No.1.

“The first couple of holes caught me off-guard actually quite a lot. I was quite nervous standing over the first tee shot and then it took me a few holes to get over it.

“It’s amazing, we’re not playing for money this week obviously. We are playing for a medal and you’re here for kind of playing for free.

“But my point is, is that it feels totally different.”

Day attributed the nerves to the reality of representing his country, rather than just himself.

“It was like, this is the first time I actually felt like I was representing a nation, which is kind of strange to say because we always talk about how we’re on the Tour and we play the PGA Tour all the time,” he said.

“We’re playing good golf and that represents your country.

“But it wasn’t like today when I was wearing these colours.

“I felt very honoured after the first couple holes to be able to play here.”

Day’s belated Olympic debut came eight years after he opted out of the Rio Games due to fears over the Zika virus, a decision he has long regretted.

Countryman Min Woo Lee – also on his Olympic debut – was bitterly disappointed to open with a five-over 76.

“It was pretty garbage but … sometimes you’re going to go a little bit backwards to go forwards,” said Lee, who had the worst possible start to his round when he hit into the water on the opening hole.

“I’m working on a couple things and there was a lot of good out there.

“But it’s just unfortunate as a player.

“You always want to play your best but sometimes the results don’t show that, especially at a course like this where it’s pretty tough and penalising.”

Chile’s Joaquin Niemann, Emiliano Grillo from Argentina and South Korean Tom Kim were tied for third at 66, a shot clear of a group of eight at four-under 67 including world No.1 Scottie Scheffler and Spanish superstar Jon Rahm fresh off his maiden LIV Golf title in the UK.

Australia is yet to win an Olympic medal in golf, which was added to the Olympic program for the Rio Games in 2016.

Minjee Lee and Hannah Green will contest the women’s tournament later in the Games.