Before any of the more than 11,000 athletes gathered for the Summer Olympics so much as set foot in Maracana Stadium for the Opening Ceremonies on Friday night, there is perhaps a single clarity about the competition over the ensuing two weeks: At some point, an athlete will step to the line or up on the blocks at the start of a race, look to his or her side to find an opponent from Russia and wonder: Is this a fair competition?

Despite a withering report of widespread, state-sponsored doping that has left the legitimacy of results for the 2014 Winter Olympics in doubt, Russian athletes are here. These Games already won’t be able to escape the legacy of those from two years ago in Sochi, Russia, because the International Olympic Committee declined to bar the entire Russian team from competition. Though the bodies that govern individual sports have banished some Russian athletes — notably, the entire track and field team — others will contend for and undoubtedly win medals.

That the Rio Olympics will be staged in a country in political and financial disarray has been expected for much of the year. Now, too, the competition will be held in uncertain circumstances. With two days to go before the Games officially begin, athletes are grappling with an array of dynamics: relief that banned cheaters won’t sully the results, concern that the fields in some of the Russians’ strongest sports will be thinned and frustration that dopers continue to appear around every corner, some backed by elaborate schemes to dodge the rules.

“You want to be able to compete on an even playing field,” American swimmer Michael Phelps, who has won more medals than any Olympian in history, said Wednesday. “We’ve had this problem for, God, how many Olympics now? It seems almost every time there’s something that comes up. And it’s sad. That’s really what it is. It’s really sad that we can’t control it, that somebody who is in charge cannot control this.”

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Even in the final days of preparation, there is confusion as to whether some of the more than 110 banned Russian athletes will actually be allowed to compete. FINA, the international governing body that oversees swimming, issued a statement disputing reports that Russian swimmers Vladimir Morozov and Nikita Lobintsev — initially barred from competing here — would be reinstated. Rather, with the swimming competition set to begin Saturday, the pair will have their case heard before a three-member IOC commission.

So with two days to go, the fields are not even set. Should the federations that oversee individual sports decide to ban more Russian athletes, others would be called to replace them. The Australian women’s gymnastics team, for instance, finished fifth in an Olympic test event in which the top four countries gained berths. Should a qualified team be banned, the Australians could compete.

“Our girls are on standby,” said Kitty Chiller, the head of mission of the Australian Olympic team. “We’ve investigated leotards for them, should they make it, because it’s not an easy exercise. We’re ready, and I think they’re ready as soon as it is announced — if it is announced.”

This issue looms over the Games because Yuliya Stepanova, a Russian track athlete, and her husband, a former Russian anti-doping official, exposed widespread doping by their compatriots in a 2014 German documentary. In May, the New York Times reported that Grigory Rodchenkov, the former head of a Moscow anti-doping lab, was directed by the government to trade out urine samples of Russian athletes — swapping dirty for clean.

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That led the World Anti-Doping Agency (WADA) to launch an investigation into the Russian team; it appointed Toronto professor Richard McLaren to head the probe. McLaren’s report, released July 18, laid waste to virtually the entire Russian operation, accusing the government of helping with doping from at least 2011 to 2015 — not just in Sochi but dating to before the 2012 Summer Olympics in London.

Less than a week after the report’s release, however, IOC President Thomas Bach said the IOC would not punish the entire Russian team, nearly 400 athletes strong. Rather, he turned the responsibility for such a decision over to the 28 international federations that oversee each sport.

Thus, there won’t be Russian weightlifters, and much of the Russian rowing team has been banned as well. But there is a clear suspicion among some competitors that athletes who are using performance-enhancing drugs will compete. As Phelps said: “I don’t know if I’ve ever competed in a clean sport.”

“I would have to say, from a personal standpoint, it’s very concerning to me that our governing bodies have dropped the ball in many ways on this,” said Phelps’s coach, Bob Bowman, who also serves as the coach of the U.S. men’s swimming team. “The system is broken, and it has to be fixed.”

Statements like that leave both the IOC and WADA defending their actions as the Games approach.

“I don’t think the system is broken,” WADA President Craig Reedie said at a news conference here. “Certain parts need revision. We’ve identified the bits we want to look at: cheating, bribery in laboratories, the consequences of noncompliance. . . . We are encouraged by the support from athletes.”

Meanwhile, Russia vehemently opposed any bans, arguing that innocent athletes would inevitably be barred. Anastasia Nazarenko, a 23-year-old who won gold for Russia in rhythmic gymnastics four years ago, said the scandal has been difficult on Russian athletes.

“Sometimes foreigners don’t understand the Russian mentality,” said Nazarenko, who is at the Rio Olympics as a commentator. “We will never give up. We will fight to the end. . . . We don’t want to be discriminated based on nationality. It should not be collective responsibility. It should be personal justice.”

Thus, the Russians are far from issuing a mea culpa. Konstantin Vybornov, the head of the information department at the Russian Olympic Committee, said he does not yet know the final roster of Olympic athletes or when they will find out. Some Russian athletes whose eligibility is unclear are in Rio, but they have not been accredited for the Olympics and can’t access the Olympic Village housing.

“Nobody knows the final list,” he said. “It is not a comfortable situation.”

Vybornov also disputed the credibility of the whistleblower for the McLaren report.

“What is the proof?” he said. “. . . The trust in Russian sport, the trust in Russian athletes, the trust in the whole system is about zero. But it’s not as bad as it’s described.”

Indeed, there is an understanding that doping is not solely a one-nation issue.

“It’s not just Russia,” said Tricia Smith, the president of the Canadian Olympic Committee. “We know that there are doping issues around the world. There’s always people that are going to try to cheat.”

That, then, is the other clarity about these Olympics, and before the Games begin neither athletes nor officials nor fans can escape it.