Britain's Paul Casey says he feels an "immense sense of responsibility" to win a medal after starting his Olympic debut with a four-under-par 67.
Casey, 44, is four shots behind first-round leader Sepp Straka, who is a shot clear of Thailand's Jazz Janewattananond in second place.
Austria's Straka, who is ranked 161st in the world, sunk eight birdies in a bogey-free round of 63.
"I've got to try and deliver as much as I can," said Casey.
"There are other sports which are the traditional - as we would say - Olympic sports we are used to watching, and we are not under the spotlight. But I feel there is an immense sense of responsibility."
Casey was one of 27 players who finished their round earlier on Thursday before the rest of the 60-man field was forced off the course by a thunderstorm at the Kasumigaseki Country Club.
Tommy Fleetwood, Britain's other representative in the men's event, finished one under upon resuming his round after the storm, as did his Irish playing partner Shane Lowry.
Fleetwood was three-under heading to the last but a double bogey at the 18th pushed him outside the top 30.
Lowry's team-mate Rory McIlroy finished two under on 69, as did Open champion Collin Morikawa of the United States.
Belgium's Thomas Pieters and Mexico's Carlos Ortiz are tied for third place behind Straka and Janewattananond after each posted six-under opening rounds of 65.
'Pride - but no pressure,' insists cool Casey
Former world number three Casey is bidding to keep the men's gold medal in British hands after Justin Rose's victory at Rio 2016.
Casey has finished seventh or better in three of the past six majors and came tied 15th at the Open Championship at Royal St George's earlier this month.
On playing at Tokyo 2020, Casey said: "I'm really proud, that's the coolest thing. I've thought about it for so long.
"Team GB don't put pressure on you [to win a medal], it's never about that. It's not been mentioned once that we need to deliver.
"But you feel that with the other athletes, because everyone else is performing so brilliantly, I want to perform and live up to their expectations as well."