Melbourne, Australia  - Serbian, Croatian and Bosnian fans have threatened to ignite a Balkan boilover during early rounds at the Australian Open.

After a mini-riot on opening day in 2007, when 150 people were ejected from Melbourne Park and a police capsicum spray attack in the stands in 2008, this year's trouble was tame.

Tensions spilled over later at the beer tent with a few punches being thrown and several participants frog-marched out of the grounds by police officers.

Verbal exchanges began in the stands of a minor showcourt as Croatian Marin Cilic beat Serb Janko Tipsavevic in the second round.

Rising Croatian Cilic called for calm after being heaped with abuse, while Tipsarevic appealed to his supports to stop the stream of insults.

"Both the Serbs and the Croats were a little bit incorrect, singing songs that had nothing to do with the tennis but to do with the history we have between each other," Tipsarevic said.

"They said a couple of bad words, so he said to be quiet and not talk about bad things," number 20 Cilic told Melbourne's Herald-Sun. "The things they were saying was really not nice. It was tough to play, of course. I knew from the beginning it was going to be like that. There's not much you can do."

Cilic said that Croatians in the stands didn't help matter by hurling insults of their own.

To English-speakers in the crowd, the heated words were just so much gibberish. "You don't want to know," Cilic said of the vocabulary on offer.

In the first round, American Taylor Dent copped it from supporters of Bosnian-born American Amer Delic, with Dent complaining five times about the chanting against him as Delic finally won.

"I've played a lot of Grand Slam tennis, and I can tell you their behavior wouldn't be allowed anywhere in the world," Dent fumed. "It was out of control. They were interrupting serves. They were chanting during line calls and even in the middle of a point."

Australian Open officials vowed to take a tough line on potential racial clashes, which have been growing around the grounds in multi- ethnic Melbourne.

At a match last year, police waded into Greek fans to pepper-spray one troublemaker and ended up hitting up to 10 innocent spectators. This year, the weaponry reportedly has been toned down for close- quarters work. (dpa)