Rory McIlroy's lack of cap biggest head-scratcher despite world No 161 Sepp Straka leading in Tokyo

Why was Rory McIlroy not wearing a hat? While that question presented the biggest head-scratcher for many on the first day of the Olympic male golf tournament, the sight of Austrian Sepp Straka on top of the Tokyo leaderboard was just as perplexing for observers of the PGA Tour.

Certainly, Paul Casey would not have expected to shoot a sparkling 67 at the Kasumigaseki Country Cub and find himself four behind a player ranked 161st in the world who has missed six of his last seven cuts.

Out in the first group, Straka - urged on by twin brother Sam, who is acting as caddie in these Games - set a furious pace, making four birdies and five pars on the front nine and repeating this scoring pattern on the inward half. “It was awesome, a dream come true for us to be out there together and we had a blast,” Sepp said. “It was one of those days that will be in my memory forever.”

On eight-under, Straka is one clear of Jazz Janewattananond - the Thai who walked away from the sport in 2016 to train as a monk - with playing partner Thomas Pieters one further back alongside Mexian Carlos Ortiz.

The earlier starters were spared the 2.5-hour rain delay caused by lightning, but Pieters had other distractions. The Belgian battled through the effects of a virus - not Covid-19 - to shoot a 65 and finally once more resembled the golfer who was so impressive five years ago on his first and so far only appearance start in the Ryder Cup.

“Just to medal would give me legendary status in my country,” Pieters said.

For Casey, the experience was different to what he expected. “I thought it would be like a Ryder Cup, but there was not an ounce of nerves, just 100 per cent excitement,” Casey said after a round featuring five birdies and a bogey. “I’ve thought about this for so long, but you can't class yourself as an Olympian until you've actually started your competition - especially with the Covid tests going on. It was really, really cool and I’m just so proud.”

While Casey’s fellow Team GB player Tommy Fleetwood had to be content with a 70 after seeing his efforts marred by a visit to the water on the 18th and a double-bogey, McIlroy was solid in a 69 containing four birdies and two bogeys. It left him in a tie for 20th, but on a course which should plainly suits, the Irish player could be poised for a notable final 54 holes.

Despite his seeming indifference coming into the event, McIlroy seemed to enjoy his debut, even going so far as saying: “Olympic golf can get up there with the majors.” So this was a different McIlroy, although what viewers found most unusual about the 32-year-old was his lack of a cap.

Inevitably, the suspicion was raised that this Nike-sponsored individual did not fancy wearing the logo of another sports apparel giant on his head (Adidas, who sponsor the Irish Olympic team). Yet McIlroy revealed there is another reason for going scalp free. "I have such a small head,” he told the Golf Channel.

Why should that matter? Well, at the 2016 Ryder Cup in Hazeltine he also played cap-less and explained it thus. “I have a pea head and the hats are way too big for me. I need them custom made.”