Juan Martin del Potro bounced the ball, waiting to try to serve out his second-round match at the Olympic tennis tournament.

Argentine fans sang for him. Brazilians chanted for his opponent, seeking to drown out their rivals. Boos and whistles and shushes echoed around center court.

"Please," the chair umpire said into his microphone, imploring spectators for some quiet.

There has been little of that at the Olympic Tennis Center in Rio de Janeiro. While crowds are sparse for many sports, they've been healthy for tennis — and often far louder than the number of folks in the stands would suggest.

Wimbledon, this is not.

"It's different when you play in Europe — the people are more calm," Brazilian player Thomaz Bellucci said. "In Brazil, people get crazy, and it's very nice."

Jeering is common, and fans often yell out in the middle of a point or as a player is about to serve — no-no's in tennis.

"Sometimes we don't know exactly when it's OK to cheer," acknowledged Mauricio Vieira, a 40-year-old Brazilian fan who attended del Potro's first-round match against top-ranked Novak Djokovic. "You have to understand, our point of reference is soccer."

And that point of reference is to be raucous — all the time.

"We are not used to these kinds of sports, so we watch it as we know it," said 34-year-old Vanessa Pessoa, who works in project management. "We make a lot of noise because it's who we are."

Players have mostly enjoyed the atmosphere, even when it's gone against them. Some of the loudest roars so far came on the second-largest court, where the Brazilian doubles team of Bellucci and Andre Sa upset second-seeded brothers Andy and Jamie Murray of Britain in the first round Sunday.

"It beats playing in front of two men and a dog," said Jamie Murray, whose regular doubles partner, Bruno Soares, is Brazilian.

Other than rooting for Brazilian players and against Argentines — as del Petro experienced in his first two matches — the home fans can be unpredictable in their allegiances. The objects of their cheers and jeers can shift from set to set, game to game and even point to point.

"They play with the players: If you win the point, they start screaming," Bellucci said after his first-round singles match.

They seem to like the big names, but also underdogs. They love showmanship and, perhaps not surprisingly, passion. Serena Williams earned her biggest roars in her first-round match for her loudest bellows of "Come on!" But when she smashed a racket in her second-round match, boos rained down.

"It's different than your average tennis crowd," Williams said. "I like it."

In sister Venus' first-round match, the crowd swung to the side of her diminutive opponent, 61st-ranked Kirsten Flipkens of Belgium, stoked by her drop shots and her will to rally from a big deficit.

"Goosebumps all over the place, from the first to the last second," Flipkens said.

"They just enjoyed my game because, I think, I'm a little girl trying to do some special stuff to get a chance to beat a champion like that," she added.

During that match, some spectators tried to start a "U-S-A!" chant that was quickly drowned out by boos. The Brazilian crowd directed some of its loudest derision toward Bellucci's first-round opponent, Dustin Brown, the dreadlocked German who's typically a fan favorite.

But when Brown twisted his ankle and then tried to play through the injury, cheers poured down from the stands.

Bellucci said he had never experienced anything like the atmosphere that day on center court. Home matches in Davis Cup — the head-to-head competitions between men's national teams — generally draw 2,000-3,000 fans, Bellucci added.

Even non-Brazilian players have little to compare this to. The only similar setting Flipkens could remember in her career was the 2006 Fed Cup final — the women's national-team competition — that Belgium hosted.

France's Jo-Wilfried Tsonga, who had the crowd both for and against him in his first-round win, said the atmosphere was far livelier than at the 2012 London Olympics.

"They give us emotions, so that's good," he said.

Rio hosts a clay-court tournament in the winter that draws a handful of top players. At the Olympics, though, many spectators seem to be novices at watching tennis — but not at sports fandom.

"Who is not shouting has problems. It's too much fun," said Clarisse Lopes, a 30-year-old psychologist attending her first pro tennis tournament. "It's not every day we get to see a Djokovic or Serena Williams play in our country. This is such a treat, and we don't have a better way of expressing it."

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AP Tennis Writer Howard Fendrich and Associated Press writer Adriana Gomez Licon contributed to this report.