Ten days of athletics start at the Olympic Stadium on Friday with all Russian athletes bar one banned because of allegations of state-sponsored doping.
American athlete Devon Allen is disappointed he will not have the chance to test his prowess against Sergey Shubenkov, the reigning 110m hurdles world champion.
"As an athlete you always want to compete against the best," said Allen, a medal contender in Rio who posted the second fastest time of the year of 13.03 at last month's US Olympic trials.
Pole vaulter Sam Kendricks, a second lieutenant in the US Army reserve, said he was saddened that clean Russian athletes had not more time to fight their case against exclusion.
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"The Russians have always had a history of being very immaculate in the vaults," Kendricks told a press conference.
"I can only express that I'm kind of disappointed that this issue didn't come to light sooner, that the clean athletes could have been able to shine through and have more due process in that respect.
Russian athletes were suspended from international competition last year after the World Anti-Doping Agency (WADA) published a report which alleged "state-sponsored" doping.
The International Association of Athletics Federations (IAAF) extended the ban to cover the Olympics after ruling that the criteria for reinstatement had not been met.
Kendricks said he understood the reasons for the hardline stance by the IAAF.
"This is something the sport has to go through to put more light on this issue so that everybody is held to the same standard," he told reporters.
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