A few days earlier, at the New York Grand Prix, this amiable 27-year-old from Phoenix, Arizona, had run the third fastest 110 metres hurdles of all time, 12.84sec, beating world champion and fellow American Grant Holloway in the process.

And as he sat in the pre-event press conference alongside home hurdler Pascal Martinot-Lagarde, he was asked as many questions about American Football as he he was about athletics, for he was already booked on a three-year contract with National Football League team Philadelphia Eagles.

First, however, he was looking forward to the World Athletics Championships in Eugene, close to where he had first excelled as an athlete and American footballer for the University of Oregon.

As things turned out, that final athletics rendezvous - for now - turned out to be traumatic. 

His start in the 110m hurdles final was minutely close to being perfect - but unfortunately it was a thousandth of a second too fast rather than too slow.

The local hero was disqualified after registering a reaction time of 0.999sec, 0.001 sec inside the legally allowed requirement of  0.1sec.

In Paris he had joked about his and Holloway's relative strengths. 

"If I had Grant’s start I probably would have run 12.6 by now," he said with a grin. 

"But he’s the greatest in the history of the 60m hurdles and probably the greatest starter for a long time.

"Me and Grant have spoken about that. I said, 'hey, teach me how to start', and he said 'teach me how to do your last four!'"

At Hayward Field, he was agonisingly close to getting the greatest start he ever had. Friends and supporters raged on his behalf. But rules is rules.

Allen has managed his switch to the NFL adroitly with a team that, yesterday in Glendale, Arizona, contested and lost the Super Bowl to Kansas City Chiefs. 

For all his talent and application, however, Allen - as a member of the Eagles' practice squad - was not an active player in the grand finale.

As he was at pains to point out in Paris, he has not quit athletics. 

Allen, who finished fifth in the Rio 2016 final and fourth in the Tokyo 2020 final, plans to divide his time between track and playing field.

"Everybody is saying I’m quitting. I’m not quitting," he said. 

"During the football season I’m going to play football. And once February comes along and we’ve won the Super Bowl hopefully I’m going to come back and start training for track and get ready for the major championships."

It nearly came to pass. And if there is any sporting justice there will doubtless be triumphs ahead for him.

If attitude has anything to do with anything, it will surely come to pass. Asked in Paris if his 12.84sec clocking had come as a shock, he replied: "I always believe I can do it. That’s the reason why I am here in track and field because I believe I can be the best in the history of the sport."

All of which is to illustrate just how good you have to be to bridge the gap between track and field and American football.

Allen has followed in the tracks of some illustrious forbears in this respect.

Ollie Matson, booked for the then NFL side Chicago Cardinals, held off signing so he could compete at the Helsinki 1952 Olympics, where he won 400m bronze and 4x400m silver.

He was inducted into the Pro Football Hall of Fame in 1972 after a stellar 14-year career.

Willie Gault, who won 200m bronze in the first World Athletics Championships in Helsinki in 1983, as well as gold in the 4x100m, where the United States broke the world record, had an 11-year career in NFL and won the 1985 Super Bowl with the Chicago Bears.

Allen's switch as a 110m hurdler followed the path set by Renaldo Nehemiah, who set three world records, the last of them 12.93sec, but was denied the opportunity to compete at the Moscow 1980 Olympics due to the United States boycott.

He went on to play for the San Francisco 49ers and was a member of the team that won the Super Bowl in 1984.

James Trapp, who won the world indoor 200m title in 1992, won the Super Bowl with the Baltimore Ravens in 2001.

Marquise Goodwin, who reached the London 2012 long jump final and had a best of 8.33 metres, has since had a flourishing NFL career with the Seattle Seahawks, the Buffalo Bills, the San Franciso 49ers and the Chicago Bears.

Michael Carter came close to the divine double of Olympic gold and a Super Bowl winner's ring.

Carter, whose daughter Michelle would win the women’s Olympic shot put title at the Rio 2016 Games, won shot put silver at the Los Angeles 1984 Games before signing up with the San Francisco 49ers, with whom he won the Super Bowl three times, the first of them within six months of his Olympic final.

But only one man has so far achieved that double - "Bullet" Boy Hayes.

Hayes was a superb natural athlete, and footballer, at Florida A&M University in the early 1960s. 

In 1962 he equalled the world 100-yards record of 9.2sec and became the first person to better six seconds in the 60-yard sprint, clocking 5.9sec.

In 1963, he lowered the 100-yards record to 9.1, a mark which stood until 1974. In the same year he set an unratified world record in the 200m of 20.5.

As the big beasts of the NFL circled, and his football coach Jake Gaither jealously guarded his services as the Tokyo 1964 Olympics loomed, the potential of his participation was uniquely underlined as the President, Lyndon Baines Johnson, put in a call to the coach to insist he allowed Hayes sufficient time off to train and stay healthy.

In Tokyo, Hayes repaid every atom of the President’s advocacy.

Tokyo was the last Olympics to be run on a cinder track, and Hayes was drawn in lane one in the 100m final, which had been churned up by the previous day’s 20km race walk.

He was also running, astonishingly, in borrowed shoes because one of his shoes had been kicked under the bed when he was playing around with some of his friends in the Olympic Village and he didn’t realise until he reached the track. Strange but apparently true.

He won gold in a world record of 10.06sec, although he was hand-timed at 9.9sec, making him the first man, effectively, to have beaten 10sec for the 100m.

He followed up by anchoring the United States to gold in the 4x100m relay in a world record of 39.06sec.

It was the last race he would run. At the age of 21, he moved over the Dallas Cowboys, for whom he played for seven seasons, winning the Super Bowl in 1971 and causing new zonal defence systems to be established in the NFL as no single opponent could handle his speed and power.

In the relay final, Hayes had been trailing as the baton was handed over, but in one of the most exhilarating sequences in Olympic history he produced the fastest relay leg then recorded, hand-timed between 8.5 and 8.9sec, to cross the line first.

France's anchor runner Jocelyn Delecour reportedly said to US runner Paul Drayton before the final: "You can't win, all you have is Bob Hayes." 

Drayton replied: "That's all we need."