Chambers was banned after testing positive for steroids in 2003
Dwain Chambers ran the best 100m by a British athlete this year by finishing second in 10.05 seconds in Sofia.
The 30-year-old lost his unbeaten season's record behind fast-finishing Asian record holder Samuel Francis.
The former drug cheat is under pressure from the British Olympic Association not to compete at the Olympic Games.
But Chambers showed his determination to represent Team GB in Beijing when he finished behind the Nigerian-born Qatarian who was 0.02secs quicker.
"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.