Belgium's Defline Persoon, who returned to amateur boxing so she could compete in Tokyo 2020, has lost her opening bout in the European qualifying tournament in London to Nikoleta Peta.
One of the most successful women's professional boxers of recent years -- Persoon narrowly lost her professional world title to Katie Taylor last June -- she will have another opportunity to qualify in Paris in May.
Elsewhere on the first day of the qualifying tournament, British lightweight Caroline Dubois won the first fight of her senior career against Belarus' Ala Staradub.
The 19-year-old Londoner, an unbeaten junior world and Olympic champion, outpointed Staradub of Belarus in the opening bout at the Copper Box near London's 2012 Olympic stadium.
The event, involving 342 male and female boxers from 43 countries with 77 Olympic spots available, remains on despite most other sport in Europe being cancelled due to the coronavirus outbreak.
Boxers from countries like Italy, the worst affected by the outbreak in Europe, have been in Britain for several weeks.
Dubois, whose 10 siblings include unbeaten professional heavyweight Daniel Dubois, has a tougher next fight against 2016 Olympic bronze medallist and top seed Mira Potkonen.
"It was an alright fight, I'll give it a C, I could have done a lot better but obviously there was a lot of pressure that was built up," Dubois said.
"Even though Mira is number one seed, she has only got two hands, two legs and one brain. If I can't beat her, I can't ever say I will be the best."
Meanwhile, Italian super-heavyweight Clemente Russo, a double Olympic silver medallist, blamed digestive problems for a no-show in his Tokyo 2020 qualifying bout with Britain's Frazer Clarke on Saturday.
The 37-year-old two-times world champion, seeking to fight at a fifth Games, told his 130,000 followers on Instagram that 'bad digestion' had kept him stuck in his room for the past 24 hours.
"I've tried to train, I've tried to react but I am somewhat weakened and they will not let me fight," said Russo, whose coach cited an abdominal problem, according to the event organisers.
The sparsely-attended qualifying tournament at London's Copper Box involves 342 male and female boxers from 43 countries and is one of few sporting events going ahead on a calendar ravaged by the coronavirus.
Italy is in lockdown as the European country worst hit by the fast-spreading virus but Russo and the Italian boxing team have been training in Britain for several weeks.
Russo said his dreams were not over, with one more European qualifying tournament scheduled for Paris from May 13-20. Whether that happens in the current climate remains to be seen, however.
"Today more than yesterday I am convinced I can do it, I am convinced I can once more take Italian pride, which at the moment is going through great difficulty, to the top," said the 2008 and 2012 silver medallist.
Clarke, a former sparring partner of professional heavyweight world champion Anthony Joshua, was given a walkover.
The 28-year-old is making his third attempt to qualify for a Games after losing out to eventual gold medallist Joshua in 2012 and Joe Joyce in 2016.