Ethnic tensions boiled over at the Australian Open on Friday when dozens of Serbian and Bosnian fans began fighting after the match between Novak Djokovic and Amer Delic.

Police arrested two men and ejected another 30 people from the grounds after the rival supporters began throwing chairs and trading punches and kicks.

One woman was knocked out after being hit on the head by a chair although police later said she had not been seriously injured.

The fighting began in a garden outside the main centre court arena then flared up again when they were escorted outside Melbourne Park.

Inspector Chris Duthie told reporters two men were charged with riotous behaviour and discharging a missile while another man was issued with an on-the-spot fine.

"The matter was dealt with very quickly and effectively by the police and we expect no more problems," Duthie said.

"We have a very effective police presence here. You can't stop everything happening but we will make sure this sort of thing doesn't happen (again)."

It was the third year in a row that the Australian Open has been marred by ethnic violence.

WILD BRAWL

In 2007, 150 Croat and Serb fans engaged in a wild brawl, hitting each other with bottles and flagpoles.

In 2008, police used capsicum spray on a group of rowdy Greek supporters and just two days ago, two people were evicted after scuffling at a bar.

Tennis Australia chief executive Steve Wood said tournament organisers were treating the incidents seriously.

"This behaviour should never have happened in the first place," Wood said. "We are taking additional security measures to protect the integrity of the nation's much loved event."

Former three-times champion Roger Federer said he was appalled that the event was being marred by fighting.

"It's not what's supposed to happen," he said.

"This tournament works so hard all year long to make up a good event. I call it the 'Happy Slam.'

"Then you come here and you see these scenes. They don't deserve it, this tournament."

BALKAN TENSION

Trouble also flared in Croatia, which is hosting the World Handball Championship until Feb. 1, state television reported on Friday.

The mayor of the southern Adriatic city of Zadar ordered the removal of all 24 participating countries' flags from a central square because residents complained the Serbian flag was among them, it said.

Mayor Zivko Kolega urged citizens to show a "spirit of sport" in handball games in Zadar which will include the Serbian team.

The former Yugoslav republic of Croatia fought a four-year independence war in the 1990s against a rebel Serb minority. The area around Zadar was among the worst affected.

In Melbourne, there was a heavy police presence already anticipating trouble at Friday's centre court match between Serbia's Djokovic, the defending champion, and Bosnian-born American Delic.

Delic had even posted a note on his website before the match, pleading for fans to behave themselves.

"I'm really sad to hear about that," he told a news conference. "As I said a couple days ago, there's absolutely no place for that here. This is a tennis match.

"I don't know who started, who finished, or whatever... on both sides I'm sure there were guys that wanted to cause problems."

Djokovic said he was unaware of the violent clashes but said the off-field rivalries were not shared by the players.

"We have big respect for each other, Amer and myself. We know each other for a long time. He's a great guy," Djokovic said.

Serbia's Ana Ivanovic, who made the Australian final last year but was knocked out in the third round on Friday, also called for a truce between the fans.

"Sport is a great thing to bring people together, no matter where you come from," Ivanovic said.

"I really hope in the future people can end this and appreciate the tennis and the hard work we put in and just enjoy it rather than have fights."