Britain's Olympic champion ran 10 seconds dead to go through to today's semi-final. It was an immediate answer to Dennis Mitchell of the US, who had made much of his winning second round time of 10.08. Cason answered in his turn in the next heat, beating the Olympic silver medallist, Frankie Fredericks, with a personal best legal time of 9.96sec.
All the major contenders went through, including Carl Lewis. Britain's second 100m representative, Jason John, was among the qualifiers with a time of 10.31.
Mark Plaatjes, the South African who now does his marathon running for the United States, produced the second most significant run of his life to win the first men's title here, overtaking Lucketz Swartbooi of Namibia with 25 of the 26 miles gone to finish in 2hr 13min 57sec.
His most significant run will always be one which saved his life several years ago in his native Johannesburg, when he produced a turn of speed to escape a mob intent on 'necklacing' him - burning him alive with a tyre around his neck.
'It was just a matter of survival,' said Plaatjes - but here he was referring to the race. 'I just kept the same pace throughout. I knew I could be second when I broke away from the group of three, and then I made up 30 seconds in 100 metres and I knew I could win.'
Swartbooi, a librarian's assistant who led last year's Olympic marathon in the early stages but failed to finish, judged his effort better in humidity of 60 degrees. But after crossing the line in 2:14.11 he could not help but cry - presumably at the thought of how close he had been. Plaatjes embraced him, and the tears were replaced with a rueful smile.
Thus the naturalised American - whose change of nationality cost him a place in last year's Olympics - became the second gold medallist to pick up the accompanying prize of a Mercedes Benz car. The first had gone to a walker - Sari Essayah, of Finland, who won the 10km event.
Paul Evans of Britain moved up to fourth place after 20 miles before being forced to drop out with stomach cramps. Steve Jones, who finished a dehydrated 20th in the 1986 European Championships here after completing the first half of the race in a world record schedule, was a respectable 13th this time around.
Rob Denmark, who looked over-awed in the presence of the leading Kenyan and Ethiopian 5,000m runners in last month's grand prix at Crystal Palace, re-established himself as a medal contender in tomorrow's final by winning a heat that included the world silver medallist Fita Bayissa and the Olympic 10,000m champion Khalid Skah in 13min 41.55sec. Denmark found himself boxed on the final bend, but found a way round to catch and outsprint the leading group of four.
All three British 800m contenders - Curtis Robb, Tom McKean and Martin Steele - came through their first round heats, with Steele, the fastest man in the world this year, having the most difficult task in his first major outdoor championship appearance. It should prove a learning exercise.
There was a similarly comforting story in the women's 3,000m, where Sonia O'Sullivan of Ireland, and Britain's trio of Yvonne Murray, Alison Wyeth and Paula Radcliffe all reached tomorow's final. -
Michael Johnson, unbeaten over 400m since February 1989, looked ominously easy in negotiating the first of four one-lap rounds, although his compatriot Butch Reynolds produced the fastest time of the day, 45.34sec.
'I think I'm ready to give Michael Johnson a much better race than he's had all season,' he said. 'I feel joy at being here.'
But the joy of the Olympic champion, Quincy Watts, has been diminished by a bruised back which he suffered when he slipped in the shower on Thursday. How galling for David Grindley that he is not fit to challenge for an individual 400m medal in such promising circumstances. His British team-mates, Du'Aine Ladejo and Ade Mafe, progressed safely.
Among those who didn't make it through the first round of the 100m was Fredrick Canon of Nauru. The sole male entrant from that small South Pacific island ran a time of 11.72sec, which placed him above Bakar Chehou Inzoudine of Comoros and behind the other 63 finishers.
Canon was obliged to run barefoot, having carelessly left his shoes back in the athlete's village, and said: ' I have tried to do my best without them.'
A man can do no more.