Serb Novak Djokovic downed Bosnian-born Amer Delic to reach the last 16 of the Australian Open but the champion's victory was marred when ethnic tension burst into violent crowd scenes on the grounds.
Groups of rival fans hurled chairs at each other in the beer garden outside centre court when simmering tension boiled over under the hot Melbourne sun.
One woman was knocked cold when a chair struck her head.
Police arrested two men and ejected another 30 people from the grounds after the rival supporters threw chairs and traded punches and kicks.
The fight spilled outside the stadium before police got it under control.
Tensions between rival ethnic factions from the former Yugoslavia have become an unfortunate feature at the season's opening grand slam -- once dubbed the Happy Slam.
Two years ago 150 fans were ejected after violent brawling at the tournament.
The tension had been rising all week, reflected in minor incidents of rowdiness at matches and Delic had appealed on his website for calm, but to no avail.
"I'm really sad to hear about that," Delic said. "There's absolutely no place for that here. This is a tennis match.
"As I'm sure you all saw at the end, I mean, Novak and I are friends.
"We're both competitors, obviously. In the end it was a fair match, and there was no reason for such things," he said after the 6-2, 4-6, 6-3, 7-6 defeat.
"There's only so much we can say. You can't stop and control everybody. Somebody gets drunk, you know, they want to cause trouble. It happens."
The violence overshadowed action at Melbourne Park, which featured world number three Dinara Safin hitting her way into the last 16, her 6-2 6-2 win over Kaia Kanepi a triumph of force over finesse.
The third seed was joined in round four by fellow Russian Nadia Petrova. The 10th-seeded Petrova sauntered through in double-quick time when Galina Voskoboeva quit after losing the first set 6-1. The Kazakh retired with a back injury.
Safina advanced effortlessly with an easy victory over the Estonian 25th seed to reach this stage for the first time.
"There's a first time for everything," she beamed. "Twice I lost in the third round. I was like this today: 'this is the third time -- should be the luckiest one'.
In truth, she needed little luck and bludgeoned her way to victory in 66 minutes
Top seed Jelena Jankovic advanced to the fourth round after overcoming Japan's Ai Sugiyama 6-4, 6-4 in a baseline tussle.
The Serbian will next play France's Marion Bartoli, who beat the Czech Republic's Lucie Safarova in three sets.
In the first night match, on-again-off-again Australian Jelena Dokic's dream run continues.
The wildcard's opening round victory over Tamira Paszek was her first Melbourne Park win in a decade and the former world number four followed that up with a second round win over Russian 17th seed Anna Chakvetadze.
She will again ride a wave of patriotic centre court support when she meets Danish 11th seed Caroline Wozniacki.