Titmus lived up to the billing of her nickname the "Terminator" when she chased down Katie Ledecky in the 400m freestyle on Monday to win one of the most anticipated races of the Games.

Doncic scored 48 points in his first Olympic appearance to tie for the second most ever in men's basketball history, leading Slovenia to a 118-100 victory over Argentina.

Titmus trailed by nearly a full body-length at the halfway mark of the eight-lap race before erasing the gap and finishing in three minutes 56.69 seconds. It was the second-fastest time in history, surpassed only by Ledecky's world record of 3:56.46 from the 2016 Rio Games.

"It's probably the biggest thing you could pull off in your sporting career," Titmus said, "so I'm over the moon."

Ledecky lost an individual Olympic final for the first time after winning the 800 free at the 2012 London Games, then capturing three more golds in the 200, 400 and 800 free at Rio de Janeiro five years ago.

Doncic scored 31 points in the first half, putting him on pace to break the Games' scoring record of 55 points by Brazilian Hall of Famer Oscar Schmidt in 1988. He ended up tied with Eddie Palubinskas, who had 48 for Australia in the 1976 Games in Montreal.

"I don't care about records," he said.

"We got a win and that's what we came here for."

Caeleb Dressel started his quest for six swimming gold medals by leading the United States to victory in the men's 4x100m freestyle relay.

Dressel, who has been hailed as the successor to Michael Phelps, gave the US the lead in the opening leg and his teammates made sure it stood up.

The other winners in the pool on Monday were Great Britain's Adam Peaty in the men's 100m breaststroke, and Canada's Maggie MacNeil in the women's 100m butterfly.

Nikita Nagornyy nailed his floor routine with the entire gym watching, and the Russian men completed their long climb back to the top of the gymnastics world by edging Japan and China in the team final.

It's the first Olympic title for the Russians since the 1996 Atlanta Games.

Youth was served at the debut of the Olympic skateboarding event for women.

Three teenagers swept the podium at the street event with 13-year-old Momiji Nishiya of Japan taking the gold medal, 13-year-old Rayssa Leal of Brazil getting silver and 16-year-old Funa Nakayama of Japan winning bronze.

Naomi Osaka and Novak Djokovic easily advanced to the round of 16 at Ariake Tennis Park. Osaka crushed 49th-ranked Viktorija Golubic of Switzerland 6-3 6-2.

Djokovic dispatched 48th-ranked Jan-Lennard Struff of Germany 6-4 6-3 to move one step closer to claiming the fourth title of a golden slam - victories in all four grand slam tournaments and Olympic gold in the same calendar year.

Australian Associated Press