With reigning Olympic and World champion Cathy Freeman taking a year off, it looked like Merry - fastest in the world this year - might miss out on the chance to claim her world crown.
But she is back in contention for gold after her commanding victory at Crystal Palace last night.
Her winning time of 50.67seconds was over a second outside the season's best she set before her illness at the start of June, but it was encouraging to see her win so convincingly ahead of America's Monique Hennagan and Mexico's Ana Guevara.
She admitted: "That was a make or break race for me. It was fully for the purpose of whether I would travel to Canada or not.
"If I had not run that well, that would have been time to sit down with my coach Linford Christie and decide whether to go or not.
"But I am happy to have won and while I'm still not 100 per cent it showed I can still run extremely well. I've now got two weeks to try and get another second off my time, but I know I can do that." Backley's dream of landing the World title is also back on track after he produced his best throw for nine years to send a warning to his old rival Jan Zelezny.
The 32-year-old from Chiselhurst in Kent produced a massive final round throw of 90.87 metres to win the competition and give him a boost as he tries to land his first ever World Championship gold.
"I love competing at Crystal Palace," said Backley, who was left in tears at last year's Sydney Games after Czech Zelezny conjured up another last gasp throw to snatch the Olympic title from his grasp.
"Palace is like my living room. I'm so comfortable here. Now what I have to do is refigure that kind of feeling in Edmonton."
Olympic champion Maurice Greene won the 100m showdown with Dwain Chambers in a time of 9.98secs, but it was the Belgrave Harrier's performance in the 200m which provided the most encouragement.
Chambers produced his fastest ever time over the distance, 20.31secs, to beat fellow British medal hopes Christian Malcolm, Marlon Devonish and Darren Campbell.
He will now double up in Canada taking the final spot in the British team from Campbell, who has not recovered from a hamstring injury.
The Olympic silver medallist was left distraught after he finished last confirming his fears that he will not be ready in time.
He left immediately after the race, but it is understood he has already indicated to British team officials that he won't go to Edmonton even if he is offered a place in the 100m relay squad. Chambers said: "I was in the same situation as Darren last year. He has to keep up his confidence. When you've won a silver medal you don't suddenly lose speed overnight. He will come back from this."
In the 400m, 1998 European champion Iwan Thomas missed out for the second year running after he failed to make the official World Championships qualifying time in the race dubbed the battle of Britain.
Mark Richardson, who won last week's Norwich Union World Championship trials in Birmingham, stormed from behind to win in a time 45.22secs.
It was way inside the qualifying mark of 45.72secs and repaid selectors for their faith in picking him for their provisional team last week. Thomas finished 10 hundrethsof a second outside the cutoff-time and will now only go as part of the 400m relay squad.
World indoor champion Daniel Caines, still on the way back after injury, will join Richardson in the team having already achieved the qualifying time earlier this season.
The third place could be taken by Jamie Baulch, who is eligible after achieving the standard last year.
Olympic bronze medallist Kelly Holmes also booked her place in the team as she roared underneath the two minute cut-off point in the 800m behind Fabiane Dos Santos and Olympic silver medallist Stephanie Graf.
And Jonathan Edwards again showed his younger British rivals Larry Achike and Phillips Idowu the way home, winning the triple jump with a first-round leap of 17.29 metres.
