Grant Hackett believes it would be 'mad' to write off five-times Olympic champion Ian Thorpe's bid to swim at the London Games even if his times are a cause for concern with the Australian trials just a few weeks away.

Hackett, who won back-to-back 1,500 meters golds at the Sydney and Athens Olympics, said his fellow Australian had left himself with an uphill task after coming out of retirement only this time last year.

Thorpe swam 52.28 seconds in the 100 freestyle at a meet in Switzerland at the weekend, a time that would be highly unlikely to secure him even a relay berth at the March 15-22 Australian trials.

"I think the times he has been swimming, it's clearly a bit of a concern. I don't think anyone can deny that," Hackett told Monday's Sydney Morning Herald.

"You can't write him off yet, and I think people would be mad to.

"But at the same time I don't think anyone can deny, and I don't think he could deny, that seeing a 52.2 on the scoreboard only 18 days out is a bit of a concern and probably raises not just a few eyebrows around him, but also his own.

"But he's the sort of guy who will get on with the job at hand. He knew this was going to be a tough challenge in the relatively short time frame he had, so we'll just have to wait and see.

"But that's the great thing, everyone is waiting. There is so much attention around this, and if he does qualify it will be huge, and I really hope he does."

Hackett said the fact that Thorpe had swum quicker times previously indicated that his Zurich performance was simply the result of the stage of training.

He also concurred with national swimming coach Leigh Nugent in suggesting Thorpe's best hope of qualifying for London lay in the 200 freestyle, a distance at which the 29-year-old held the world record (1:44.06) for six years from 2001.

"I've said from the start I was quite surprised the 100m was the one he was going for," Hackett told the paper.

"Even though it takes that extra bit of work for the 200m, his natural ability lies in that event, and I think that's still going to be his best chance."