US swimmer Caeleb Dressel opens his bid for a Michael Phelps-style medal haul at the Tokyo Olympics on Monday, as judo's Shohei Ono will try to keep the golds rolling in for Japan.

Dressel sat out Sunday's 4x100m heats but the United States still qualified second fastest behind Italy, and they will be favourites for the morning final.

It could be the start of a seven-title spree for Dressel, 24, whose 13 world titles have prompted inevitable comparisons with Phelps -- winner of eight golds in 2008 and 23 in total.

The pressure is on the US team to emulate the golden generation of the retired Phelps along with Ryan Lochte, Nathan Adrian and Tony Ervin, who failed to qualify.

"I think everyone who is on the team, we're going to have to pick up the pace because what they left behind is huge," Dressel said.

In the men's 100m breaststroke final, Adam Peaty is the clear favourite as he attempts to become the first British swimmer to successfully defend an Olympic title.

Peaty, who has lowered the world record five times, won his semi in 57.63 seconds to lead the timings into the final ahead of Dutchman Arno Kamminga with 58.19.

USA's Caeleb Dressel is a 13-time world champion
USA's Caeleb Dressel is a 13-time world champion AFP / Oli SCARFF

Away from the pool, three-time world champion Ono is expecting a "life-or-death" battle in the Nippon Budokan, judo's spiritual home, as he goes for his second Olympic gold.

"The Olympics is not a place for enjoyment," he said in a recent interview with public broadcaster NHK. "For me, it's a life-or-death battlefield."

After an exceptionally rocky build-up to the Games, with a year's delay and fans banned over the pandemic, Japan have enjoyed a strong start with five golds in the first two days.

As skateboarding continues its Olympic debut, women's street world champion Aori Nishimura will hope to emulate Japanese teammate Yuto Horigome, who won the men's competition on Sunday.

Britain's Jonny Brownlee will go for triathlon gold in the absence of his brother Alistair, who won the last two Olympic titles but failed to qualify this time around.

Brownlee, who took bronze in 2012 and silver in 2016, is aiming to become the first athlete to win three Olympic medals in the swimming, cycling and running race.

Elsewhere, Serbia's tennis world number one Novak Djokovic takes on Jan-Lennard Struff of Germany in round two as he pursues his bid for a first Olympic title.

The 2008 bronze medallist has won the Australian Open, French Open and Wimbledon this year, giving him a shot at the Golden Slam -- winning all four majors and the Olympics in the same season.