By Ossian Shine and Antoni Slodkowski

Olympic host city Tokyo reported a record number of coronavirus cases on Tuesday even as athletes continued to compete across the city in the fourth day of the Games, with Naomi Osaka making a surprise exit from the tennis tournament.

Tokyo, which remains under a state of emergency, reported 2,848 daily cases on Tuesday, in line with warnings by health experts that seasonal factors, increased mobility, and the spread of variants would lead to a rebound in COVID-19 this summer.

Kenji Shibuya, a former director of the Institute for Population Health at King's College London, said the surge was due to a "systematic failure of the government response to COVID-19", saying the government did not suppress transmission, ramp up vaccinations and sent the wrong message that people should stay home but also celebrate the Games.

Tokyo's virus cases are counted separately from those reported in the Olympic village. Athletes and other officials who have travelled from abroad to participate in the Games are separated from the public, and spectators have been banned from most venues.

Games organisers have reported a total of 155 cases associated with the Olympics.

Meanwhile, Osaka, who lit the Olympic cauldron on Friday to open the Games delayed by a year due to the pandemic, looked out of sorts and never settled into a rhythm as she lost 6-1 6-4 in the third round to Czech Marketa Vondrousova.

"I think it's maybe because I haven't played in an Olympics before and for the first one to be here was a bit much," Osaka said.

The Tokyo Games marked Osaka's first tournament since she pulled out of the French Open in May.

The women's tennis event has now lost both of its top draw cards after world number one Ash Barty was beaten in the first round.

SURFING GOLD

Other competitors fared better.

Brazil's Italo Ferreira and American Carissa Moore overcame challenging conditions in brilliant style to win historic first Olympic surfing gold medals.

In the women's triathlon, Flora Duffy, 33, achieved instant national hero status when she won Bermuda's first Olympic gold medal.

With about 63,000 residents, the British Overseas Territory in the North Atlantic Ocean became the smallest nation to win a Summer Olympic gold medal.

In the pool, Russian Evgeny Rylov broke a U.S. stranglehold on the men's 100m backstroke, while Tom Dean powered to gold in the men's 200m freestyle, heading a British one-two with Duncan Scott taking silver.

Olympic organisers remained on alert to monitor the impact from a tropical storm off Japan's east coast, with the nation's hot, wet and unstable summer weather patterns, a persistent concern for the Games.

At the archery venue, early rounds of the individual events were delayed by some two and a half hours due to the storm.

In Yokohama, just south of Tokyo, the sun emerged just after Canada beat Mexico for the softball bronze, playing through a steady drizzle and strong gusts of wind.

Tokyo was forecast to receive up to 31.5mm (1.2 inches) of rain over 24 hours from tropical storm Nepartak, now forecast to make landfall in the north early on Wednesday, the Japan Meteorological Agency said.