RIO DE JANEIRO >> When Venus Williams pushed one last forehand long to lose in the first round for the first time in her record five Olympic singles tournaments, her opponent celebrated as if having claimed a gold medal, dropping down on the green hard court to plant a kiss on the white five-ring logo.

This was clearly a very big deal to Kirsten Flipkens, a Belgian ranked 62nd and only once as far as the semifinals at a Grand Slam event.

Just two points away from winning on four occasions, the 36-year-old Williams faded as Saturday night’s match stretched past 3 hours and she was stunned 4-6, 6-3, 7-6 (5) by Flipkens on Day 1 of tennis at the Rio de Janeiro Olympics.

The American owns four gold medals: one in singles and three in doubles with her younger sister Serena. She owns 21 Grand Slam titles: seven in singles, 14 in doubles with Serena. But the older Williams labored at times and even showed frustration by shouting “Ridiculous!” after dropping one point.

With U.S. Secretary of State John Kerry watching from a second-row seat behind a baseline, the fifth-seeded Williams wasted a 4-1 lead in the final set and was broken while serving for the victory at 5-3.

Williams is the first tennis player to participate in singles at five Olympics, and never had failed to reach at least the third round before, claiming a gold at the 2000 Sydney Games. She also won doubles golds in 2000, 2008 and 2012.

But against Flipkens, who was making her Olympic debut, Williams failed to find the right measure on most of her strokes and wound up with 13 fewer winners. Williams, once possessor of one of the best serves in women’s tennis, hit one ace, five fewer than Flipkens.

The crowd couldn’t seem to settle on which woman it wanted to win. They greeted Williams far more loudly and warmly during prematch introductions, although perhaps that was because of her far-greater name recognition.

Later, though, they roared when Williams made mistakes. They clapped when she double-faulted. They even gave her grief for the common and innocuous practice of catching a ball toss that wasn’t good.

When one group of spectators bellowed a “USA! USA!” chant late in the third set, others responded by booing (Kerry had left by then).

Williams’ loss was part of a rough day for the United States tennis team. Seven of the country’s eight singles players were in action, and they went 2-5.

Steve Johnson beat Darian King of Barbados 6-3, 6-2, and Madison Keys defeated Danka Kovinic of Montenegro 6-3, 6-3, but otherwise there were losses for Jack Sock (who said afterward he had walking pneumonia, then got into a Twitter spat with former pro player and coach Brad Gilbert), Denis Kudla and Brian Baker among the U.S. men.

And Sloane Stephens was eliminated 6-3, 6-3 by Canada’s Eugenie Bouchard.

There were other surprises in addition to Williams’ exit: In all, three of the top six women already are gone.

No. 4-seeded Agnieszka Radwanska of Poland, the 2012 Wimbledon runner-up, was beaten 6-4, 7-5 by 64th-ranked Zheng Saisai of China.

And No. 6 Roberta Vinci of Italy — who ended Serena Williams’ bid for a calendar-year Grand Slam at the U.S. Open last September — lost to Anna Karolina Schmiedlova of Slovakia 7-5, 6-4. Schmiedlova was 4-20 this season going into that match.

Japan’s men had a strong day, going 3-0: Taro Daniel defeated Sock 6-4, 6-4; Yuichi Sugita edged Baker 5-7, 7-5, 6-4; and fourth-seeded Kei Nishikori won a point even though his racket flew out of his hand on a serve during a 6-2, 6-4 victory over Spain’s Albert Ramos-Vinolas.

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