"I only got an invite to compete at the last minute," said Chambers who on Saturday twice broke the Olympic qualifying mark in Germany.
"My legs were tired after those races," he added, "and I thought after coming into the race after a last-minute call, I would have been slower.
"Quite frankly I was knackered," he admitted after posting his best mark since 2001.
"But this is more progress and I feel I'm getting there," added Chambers whose lawyers are expected to file proceedings against his lifetime Olympic ban later this week.
Chambers who is attempting to overturn a BOA bylaw in the High Court, moved 0.01 ahead of the performance achieved by Tyrone Edgar in Geneva in May.