Two pole vaulters were ruled out of the Tokyo Olympics and Australia's track and field team were briefly forced into isolation on Thursday in a jolting reminder of the risks facing athletes at the coronavirus-hit Games.
On the eve of the athletics competition, 2016 bronze medallist Sam Kendricks and Argentina's German Chiaraviglio suffered heartbreaking disappointment when they both tested positive.
America's Kendricks, a two-time world champion, and Chiaraviglio are both in isolation and will miss the pole vault competition when it starts on Saturday.
Preparing for the Olympics has been tough in the pandemic and athletes are under extra pressure at the pandemic-delayed Games with strict biosecure measures and the constant threat of Covid.
Athletes' wellbeing has been a running concern and the women's all-around gymnastics went ahead without superstar Simone Biles, who had withdrawn because of fears over her mental health.
Eighteen-year-old American Sunisa Lee took gold, succeeding Biles as champion, with Brazil's Rebeca Andrade taking silver and Russian Angelina Melnikova bronze.
Earlier, US swim star Caeleb Dressel grabbed his second gold of the Games in the showpiece men's 100m freestyle to stay on track for a potential haul of six.
Dressel said he was proud to claim his first individual title after two relay golds in Rio and another this week in Tokyo.
"I didn't want to admit it but now I did it I can admit it, it's a lot different," he said.
Robert Finke won another gold for the United States in the men's 800m freestyle, while Australia's Zac Stubblety-Cook claimed the men's 200m breaststroke gold in an Olympic record 2:06.38.
China's Zhang Yufei won the women's 200m butterfly and China turned heads when they smashed the world record to win the 4x200m women's freestyle relay ahead of USA and Australia.
At a sweltering Ariake Tennis Park, world number one Novak Djokovic swept aside home hope Kei Nishikori 6-2, 6-0 to reach the semi-finals as he targets his first Olympic title.
The Australian Open, French Open and Wimbledon champion, on course for the first men's Golden Slam of winning all four majors and the Olympics, will face Germany's Alexander Zverev in the last four.
Switzerland's Belinda Bencic beat Elena Rybakina 7-6 (7/2), 4-6, 6-3 to reach the final, and was overcome with emotion to secure a medal.
Meanwhile organisers announced a daily record of 24 new Games-related infects, three of whom are athletes, taking the overall number of positive cases to 193.
The rising figure coincides with record numbers of new cases in Tokyo and nationwide. But the International Olympic Committee said there was no link with the Games.
"As far as I'm aware there's not a single case of an infection spreading to the Tokyo population from the athletes or Olympic movement," spokesman Mark Adams told reporters.