MELBOURNE, Australia — Defending champion Novak Djokovic survived a spirited encounter with Bosnian-born American Amer Delic in the Australian Open on Friday before crowd trouble flared outside.

The 21-year-old Serb lost a set for the first time in the tournament and had to fend off two set points in the fourth before beating Delic 6-2, 4-6, 6-3, 7-6 (4).

Moments after Djokovic and Delic embraced at the net, smiled and waved to all parts of the crowd in Rod Laver Arena, screams, water bottles and plastic chairs were hurled back and forth between the Bosnian and Serb fans who had been watching the match live on a big TV screen next to the arena.

There was no animosity on court.

“Of course, I have a big respect for Amer,” Djokovic said, to loud cheers from the crowd. “He absolutely deserves your applause and even more.

In the biggest upset so far at the season’s first major, sixth-seeded Venus Williams lost 2-6, 6-3, 7-5 on Thursday night in the second round to Carla Suarez Navarro, a 20-year-old Spaniard ranked No. 46.

Williams was broken while serving for the match and dropped the last five games.

It means there will be no Williams vs. Williams showdown in the semifinals. Second-seeded Serena Williams advanced 6-3, 7-5 over Argentina’s Gisela Dulko.

Early today, No. 8 Juan Martin del Potro of Argentina beat Gilles Muller of Luxembourg 6-7 (5), 7-5, 6-3, 7-5, remaining the only Argentine man still in the draw of the nine who started the tournament.

No. 19 Marin Cilic of Croatia had an upset 7-6 (5), 6-3, 6-4 win over No. 11 David Ferrer of Spain.

On the women’s side, Olympic silver medalist Dinara Safina reached the fourth round for the first time in seven trips to the Australian Open with a 6-2, 6-2 win over No. 25 Kaia Kanepi of Estonia.