Diving and volleyball on Saturday became the first Olympic test events to include international athletes since such competitions resumed last month, as both began under close supervision in Tokyo.
The diving World Cup, also a qualifier for this summer's Games, features more than 200 athletes from 50 countries including powerhouse China.
"We're not allowed out of our rooms, where you have to stay... - no outdoor air, no human interaction," said U.S. women's diver Sarah Bacon. "But we've been making it work."
With around 15,000 Olympians and Paralympians expected to compete, organisers are grappling with how to hold the Games safely during the COVID-19 pandemic.
Japanese authorities are determined to protect not only Games participants, but a local population that opinion surveys have shown is largely opposed to the Olympics due to the virus.
Japan is battling a fourth wave of infections, and the government has declared states of emergency in Tokyo and other areas.
During a warm-up before Saturday's men's preliminary event, the announcer scolded divers for gathering too closely, thus violating social distancing regulations, around the 3-meter springboard.
The event was also punctuated in the morning by an earthquake that shook the cavernous Tokyo Aquatics Centre, a reminder that the Games are taking place in one of the world's most tectonically active regions.
Divers expressed gratitude for the chance to compete but also frustrations that precautions against contagion denied them the chance to experience the city or get fresh air.
Participants are undergoing COVID-19 testing every morning and have to maintain health information on smartphone apps. Additionally, they are kept in a "bubble", restricting movements to between the hotel and venue.
"It would have been nice if they could have rented a bus for the team to do some sightseeing around the city," Patrick Hausding, a silver and bronze medal winner for Germany in two previous Olympics, told reporters from behind a glass window.
"To be in Tokyo and not be able to see anything is a real shame."
The diving event, originally scheduled for April, was in danger of being cancelled amid reports the International Swimming Federation (FINA) was dissatisfied with the COVID-19 countermeasures.
FINA and Tokyo organisers agreed to reschedule it for May 1-6 but the global swim body cancelled an artistic swimming qualifier that was scheduled to be held in Tokyo, and moved a marathon swimming qualifier from Fukuoka in southern Japan to Portugal.
Australia decided to withdraw from the diving due to worries about rising coronavirus cases in Japan.
Saturday's volleyball at Ariake Arena involved a friendly between the Japanese and Chinese men's teams followed by the women.
Guo Cheng, captain of the Chinese men's team, said he appreciated the chance to compete and that organisers had done a good job making it happen.
"After we arrived in Japan our food and accommodations were well taken care of and I really felt secure playing in the match," he told reporters after his team lost in five sets to Japan.
The Tokyo Olympics, delayed by a year because of the pandemic, are scheduled to open on July 23.
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