A mere glance would not suffice. A second take was required and, in some instances, a long, astonished, hard stare at the three digits on the trackside clock.
It had stopped at 9.91. The 10-second barrier had been broken. Not for 20 years had a Briton covered 100m quicker.
For those who suspected glitches and gremlins in the officials' stopwatches, the full-throated celebratory roar from the man himself, James Dasaolu, was an indication that what had happened was not an illusion, a mirage during the summer heatwave.
A month on from the British Championships, when Dasaolu became the second-fastest Briton in history, much has been written about the 25-year-old Londoner, but even more is expected of the son of Nigerian parents.
Former Olympic and world 100m champion Donovan Bailey believes the quiet and articulate sprinter can achieve great things, while Colin Jackson, 1993 and 1999 110m hurdles world champion, bristles with excitement when talking about the world being the Croydon Harrier's oyster.
But can Dasaolu push his fragile body to the limit for a possible four races in two days? Ahead of Saturday's heats in the Luzhniki Stadium, Dasaolu has not been seen competitively on a track since his blistering run at the Alexander Stadium on that warm July day.
Niggling injuries, the most recent a tight hip flexor, have troubled him, but his performance in Birmingham suggests the south Londoner could bring home the first British medal in track and field's show-stopping event for a decade.
It would fulfil the promise Dasaolu showed when, as an 18-year-old, he decided to dedicate his life to sprinting.
Indeed, Mark Lewis-Francis had predicted his British team-mate would break through in 2011, but injuries - to his calf, heel and hamstring - meant Dasaolu's headline-grabbing performances arrived later than many had anticipated.
"We've known about James for a long time," Jackson admits. "He's a precocious talent, a natural athlete. One of these tall, beautifully fluid runners. He has it all."
Former Olympic 200m silver medallist Darren Campbell talks as enthusiastically as Jackson about the sprinter.
Campbell's bronze medal in the 100m at the 2003 World Championships makes him the last British man to win a global medal in athletics' blue riband event.
"For James, it was a case of staying healthy," Campbell told BBC Sport.
"One of the keys to sprinting is trying to manage power and relaxation, which sounds crazy, but once you hit top speed you have to relax.
"It's a difficult thing to do, but over the years when I've watched James, his transition is smooth, there's nothing forced about it, which means you decelerate less.
"This year is not a flash. For the last four or five seasons he's run a quick time but then disappeared because of injuries, which means he's not cemented his name in people's consciousness - but this year he's a serious contender."
Dasaolu credits coach Steve Fudge, whom he teamed up with last year, for helping him join the sprinting elite. Dasaolu's training programme has been rebuilt, his technique refined and he receives regular medical attention.
This week, though, Dasaolu admitted he and his Loughborough-based coach have grown tired of people questioning whether his body can withstand the stresses of a major championship.
"Me and my coach are trying to get away from this tag," said Dasaolu, whose fitness has been questioned with increasing regularity since he failed to compete in the final of the British Championships and then missed the Anniversary Games.
"I don't really read the papers. What I look to is the team around me, my coach and the head coach Neil Black, so as long as these people are around me who believe in my body, those are the opinions I really care about.
"It's all about putting your body to the limits. It doesn't matter what happened before Moscow, so I am very confident I can do the rounds."
Dasaolu's 9.91 in Birmingham was his sixth personal best in his first seven races indoors and outdoors this year, but just 24 hours later his feat was overshadowed by the news Tyson Gay and Asafa Powell, two of the four fastest men of all time, had failed drug tests.
The absence of Gay and Powell plus injured reigning champion Yohan Blake has removed at least three potential medallists from the equation. It means Dasaolu will probably have to dip ahead of either Justin Gatlin or Nesta Carter - two of only three men who have run 100m quicker than Dasaolu this year - to snatch a medal.
The only other athlete to complete the distance in a better time than the Briton is, of course, Olympic champion Usain Bolt.
Former world record-holder Bailey has described the Jamaican, hoping to regain the title he lost in 2011 after a false start in the final, as "not the man - the only man", suggesting Sunday's final will be a one-man dash for gold.
But the potential presence of a Briton in the line-up against the great Jamaican in a World Championship final is one that quickens the pulse.
"James is hungry to improve and he can analyse his performance," says Jackson. "We've got one [a top-class British sprinter] and it's been a while since I've been able to say that. It's exciting."