Number 3 Serbian ace Novac Djokovic advanced to the fourth round of the Australian Open Friday as he defeated his best friend and Davis Cup teammate, Viktor Troicki, who complained of stomach problem forcing him to retire after the first set of the game.

Djokovic, who was the Australian Open Champion in 2008, expressed dismay over his victory leading 6-2 following three service breaks on the first set that lasted for 39 minutes.

"It's not the way you want to win a match, especially against one of your best friends", Djokovic was quoted as saying by The Times of India.

He broke 29th-seeded Troicki in the second, fourth and eight games and would take the opening set had his fellow Serb not called his trainor at the third changeover due to illness. The Times of India noted that the physical pain endured by Troicki had caused him 17 unforced errors.

The Serbian top ace who is expected to play either 14th-seed, Nicolas Almagro of Spain or 17th-seed, Ivan Ljubicic of Croatia in round 16 is also seen as the strongest rival for both the top two seeds Rafael Nadal and Riger Federer. Djokovic has proven to be a tough one to beat after reaching the quarter finals of all Grand Slams last year.