South Korea wasted little time picking up its first gold of the Rio de Janeiro Olympics, as the men's archery team came out on top on the first day of the competition Saturday.

The trio of Kim Woo-jin, Ku Bon-chan and Lee Seung-yun defeated the United States team of Brady Ellison, Zach Garrett and Jake Kaminski by the set score of 6-0 at Sambodromo. 

It was South Korea's fifth Olympic gold in the men's team archery event and avenged its loss to the Americans in the semifinals at the 2012 London Olympics.

South Korea is targeting an unprecedented gold medal sweep in archery this year and couldn't have asked for a better start to the quest.

South Korea's first medal in Rio came minutes before the archery title, as judo's Jeong Bo-kyeong claimed silver in the women's -48kg at Carioca Arena 2. She lost to Paula Pareto of Argentina by waza-ari (half point) in the final, but it was seen as a surprise that Jeong even reached the final.

Jeong, ranked eighth in the world, stunned No. 1 Urantsetseg Munkhbat of Mongolia in the quarters. It seemed to clear the biggest hurdle for Jeong, but the South Korean couldn't overcome Pareto, who'd beaten her in the semifinals of last year's world championships.

Earlier in the day, however, other medal contenders failed to live up to expectations.

Perhaps the biggest disappointment came at the Olympic Aquatics Centre, where swimmer Park Tae-hwan didn't even reach the final in the 400m freestyle.

The 2008 Olympic champion and the 2012 silver medal winner in that event, Park finished 10th in the heats and missed out on the final, open only to the top eight qualifiers.

Park had a strong start but quickly faded away, finishing fourth among eight swimmers in the sixth heat at 3:45.63.

Then in the seventh and final heat, five swimmers posted faster times than Park.

Park will return to the pool in the 200m freestyle heats on Sunday.

At the Olympic Shooting Centre, Jin Jong-oh couldn't defend his 10m air pistol gold medal, finishing fifth after an up-and-down final.

He would have become the first man to win back-to-back gold medals in the 10m air pistol.

While Jeong was a pleasant surprise in women's judo, Kim Won-jin, the men's world No. 1 in -60kg, was the exact opposite.

He got knocked out of the quarters, losing to Beslan Mudranov of Russia by an ippon. Then he lost his final chance at a medal by dropping to Naohisa Takato of Japan in the repechage.

Also on Saturday, the women's volleyball team defeated Japan 3-1 (19-25, 25-15, 25-17, 25-21) in the first Group A match.

It was a measure of revenge for South Korea, which lost to Japan in straight sets in the bronze medal contest at the 2012 London Olympics.

All-world attacker Kim Yeon-koung led all players with 30 points.

In fencing, Shin A-lam was eliminated in the round of 32 in the women's individual epee event, four years after losing in the semifinals on a controversial timekeeping error.

She won silver in the team event in London as her consolation prize but blew her chance at personal redemption here by losing to Olena Kryvytska of Ukraine 15-14.

Shin's two teammates, Choi In-jeong and Kang Young-mi, lost in the quarterfinals and the round of 16, respectively. (Yonhap)