Olympic bronze medallist Nathan Deakes has halved his chances of a medal at the world athletics championships, pulling out of the 20km walk because of injury to focus solely on the 50km event.

Deakes provides the best chance of a medal for Australia's injury-ravaged squad at the championships which begin at Helsinki's 1952 Olympic Stadium on Saturday, but was forced out of his preferred event with a hamstring strain.

He said he did not have time to recover for the 20km walk on the opening day but would be fit for the gruelling 50km on August 12.

"I've got plenty of time to get ready for that," Deakes said.

"I've prepared more for the 20km and have the fastest time in the world this year so it's very disappointing not to do both. I'd really set myself to do well in both events.

"So I'll just concentrate on the 50k, I'm fairly confident with that and very determined to win. I don't want to go away empty handed.

"You could say my chances are cut in half or it's a blessing in disguise, I'll be a lot fresher for the 50 now."

Deakes, 27, took over the mantle as Australia's best hope in Helsinki after defending 400m hurdles champion Jana Pittman pulled out with a back injury and pole vaulter Paul Burgess withdrew because of a calf injury.

Doctors injected Deakes with cortisone on Monday to ease the swelling in his chronic hamstring which was also aggravating his sciatic nerve.

The Geelong walker flew from his training base in Germany to the Australian team's pre-championships camp in Loughborough in England to have a scan which revealed a slight strain in his hamstring.

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