Caeleb Dressel targets second swimming Olympic gold, Biles' absence felt in all-around final

American swimmer Caeleb Dressel seeks a second gold medal at the Tokyo Games on Thursday as he pursues his quest for Olympic greatness while the absence of Simone Biles will be keenly felt in the women's all-around gymnastics event.

Some of the world's best golfers, including Rory McIlroy and newly-crowned British Open winner Collin Morikawa, will tee off early for the first round of the men's golf competition at the Kasumigaseki Country Club.

And Novak Djokovic will be aiming for a place in the semi-finals of the men's tennis singles as he faces home hope Kei Nishikori.

Dressel won his first gold after spearheading the USA to the 4x100m relay title, but reigning champion Kyle Chalmers stands in his way in the 100m freestyle in the Olympic battle of the pool between Australia and the Americans.

The American is attempting three individual events in Tokyo, with the 50m freestyle and 100m butterfly still to come and he could feature in several more relays.

"I know what my job is as I hit the water. It's not different if it's relay or individual," Dressel said after qualifying on Wednesday.

Biles's decision to ditch a second event, hot on the heels of her dramatic withdrawal from Tuesday's team event due to mental health concerns, has raised serious doubts about the American superstar's further participation in the Games.

The 24-year-old's struggles follow those of Japanese tennis star Naomi Osaka, another face of the Tokyo Olympics who lost in the third round on her return from a break to restore her own mental health.

Biles, unbeaten in all-around competition since 2013 and widely touted as the 'G.O.A.T' (Greatest Of All Time), arrived seeking five Olympic titles to equal Soviet gymnast Larisa Latynina's career record of nine.

But she withdrew after a single, shaky vault in the team competition.

After also ditching the all-around, she has four remaining individual events, if she chooses to take part.

The four-time gold medallist from Rio 2016 Biles is not hiding away -- she turned up at the gymnastics venue to cheer on the US competitors in the men's all-around final on Wednesday.

Biles told broadcaster NBC she would assess her readiness to compete on a "day-by-day" basis.

"Physically, I feel good, I'm in shape," she said. "We're going to take it day by day, and we're just gonna see."

Her very public struggles have attracted widespread support with even former US first lady Michelle Obama tweeting "we are proud of you and we are rooting for you."

Djokovic said after beating Alejandro Davidovich Fokina in straight sets in the third round on Wednesday that pressure was inescapable.

"Pressure is a privilege. Without pressure there is no professional sport," the Serb said.

"If you are aiming to be at the top of the game, you better start learning how to deal with pressure and how to cope with those moments."

Biles has hinted that the unprecedented environment of the Tokyo Olympics -- largely spectator-less and with competitors' movements strictly limited to prevent coronavirus infections -- had contributed to her anxiety.

On Wednesday, Dutch athletes in quarantine in Tokyo after testing positive for the virus revealed they had staged a sit-in strike over the harsh conditions, eventually earning the right to stand by an open window for 15 minutes fresh air a day.

"Not having any outside air is so inhuman, and it is mentally super-draining," said street skateboarder Candy Jacobs, while taekwondo fighter Reshmie Oogink called it "Olympic jail".

The International Olympic Committee said athletes in quarantine had access to psychologists in the Athletes' Village and phone helplines.