Tunisian teenager Ahmed Hafnaoui admits he surprised even himself with his men's 400m freestyle gold medal victory at the 2020 Tokyo Olympics on Sunday.

Hafnaoui, who turned 18 in December, stormed home from lane eight to win the gold in a major shock, ahead of Australia's Jack McLoughlin and USA's Kieran Smith.

The 18-year-old's time of 3:43:36 beat his personal best by almost three seconds, while bettering his prelims time by two seconds.

"Of course I was surprised with myself. It's unbelievable," Hafnaoui said at the news conference.

"I believe when I touched the wall and I saw myself first. I was so surprised."

Hafnaoui was only the second Tunisian to ever make an Olympic swimming final and claimed his country's second medal in the pool.

"I was in tears, I was in tears in my eyes because when I see the flag of my country and I hear the anthem in the background, it was great," he said. "I’m so proud of it."

Australian quartet Bronte Campbell, Meg Harris, Emma McKeon and Cate Campbell broke the world record as they won gold in the women's 4x100m freestyle relay.

Anchored by Cate Campbell, the Aussies won in 3:29:69 ahead of Canada and the USA. It was Australia's first goal medal of the Tokyo Games.

The USA's Chase Kalisz won the men's 400m individual medley, while Japan's Yui Ohashi triumphed in the women's equivalent.

Top seed Barty bundled out, Osaka cruises

Spain's Sara Sorribes Tormo stunned 2021 Wimbledon champion Ashleigh Barty in straight sets in the first round of the women's tennis singles.

Sorribes Tormo won 6-4 6-3 over the Australian, the world number one missing out on a first Olympic medal having looked well below her best.

The Spaniard controlled the baseline, while Barty only managed to land 42 per cent of her first serves.

Second seed Naomi Osaka cruised past Zheng Saisai 6-1 6-4 in her Olympic debut, returning to the court in style following a hiatus after her French Open withdrawal.

Third seed Aryna Sabalenka had no trouble dispatching Poland's Magda Linette 6-2 6-1.

Men's fourth seed Alexander Zverev eased past Lu Yen-hsun 6-1 6-3.

First-ever skateboarding gold

Skateboarding made its Olympic debut on Sunday, with local Yuto Horigome crowned the sport's first gold medallist in the men's street event.

World number two Horigome qualified in the semi-finals in sixth after scores totalling 33.75 but dominated the final with 37.18.

The Japanese finished ahead of Brazil's Kelvin Hoeffler with 36.15 and Jagger Eaton on 35.35.

Horigome landed three huge tricks in a row in the final, while American favourite Nyjah Huston stumbled down to seventh after four straight falls.

Olympic record in shooting final

Vitalina Batsarashkina won gold in the women's 10 metre air pistol shooting ahead of Bulgaria's Antoaenta Kostadinova and China's Jiang Ranxin.

The Russian tallied an Olympic record 240.3 points, finishing marginally ahead of Kostadinova on 239.4.

"I did not set a goal to win or to get into the top three or even into the final," she said. "I just set the goal to shoot like I can, to show everything that I can."

Chinese gold in diving

China won gold in the women's synchronized 3m springboard diving as Shi Tingmao and Wang Han edged out Canada and Germany.

The Chinese duo scored 76.5 with their last dive to tally 326.4 points and win China's fourth gold medal at the Tokyo Games.

The reigning world champions were well ahead of the pack, with Canadian pair Jennifer Abel and Melissa Citrini-Beaulieu grabbing silver with 300.78. Germany's Lena Hentschel and Tina Punzel claimed the bronze with 284.97.

The gold was Shi's third, having won the individual 3m platform title and this event with Wu Minxia in 2016.