MELBOURNE - Novak Djokovic once had a reputation for wilting when the physical strain became too much - not anymore.

In 2009, extreme heat forced him to retire during the quarterfinals at the Australian Open. The Serb was the defending champion and attracted some opprobrium from local media for his perceived lack of heart.

Djokovic, however, has embarked on brutal off-season conditioning programs in the last few years to ensure he does not repeat the experience, particularly at Melbourne Park.

Novak Djokovic of Serbia celebrates defeating Stanislas Wawrinka of Switzerland in their men's singles match at the Australian Open tennis tournament in Melbourne, Jan 21, 2013. [Photo/Agencies]

The benefits of the work, which he refuses to discuss publicly rather than in broad generalizations, has never been more evident than the last two years in Melbourne.

In 2012, the Serb took almost five hours to defeat Andy Murray in the semifinals and lay exhausted on the court unable to brush away moths that were fluttering around his face and arms.

Two days later, he went even deeper into the balmy Melbourne night as he and Rafa Nadal slugged it out for five hours, 53 minutes, the longest Grand Slam final recorded, before he prevailed 5-7 6-4 6-2 6-7 7-5.

Such was the exertion, both players slumped on chairs brought out for them during the presentation of the trophies.

Djokovic had to dig deep into the wells of his ironman reserve again this year when 15th-ranked Stanislas Wawrinka pushed him to his limits in a five hour, five-set thriller that ended at 1:40 am local time on Monday.

While exhausted immediately after the match against Wawrinka the world number one, however, was certain the experiences in 2012 would only help as he fine tuned preparations for his quarterfinal against Tomas Berdych on Tuesday.

"I've been in those situations before," Djokovic said.

"I remember most recently, when I won against Murray in the semis after five hours and then played against Rafa almost six hours, I know I can recover. I know I have it in me."

Djokovic's sleep has been affected by his late nights in Melbourne, often not emerging until the early afternoon, and he said after his battle with Wawrinka he already felt like he was at the end of the tournament rather than just halfway through it.

"There is no doubt I'm not very fresh. It's normal to expect after five hours of play," he said.

"But in the end these kind of matches, after five hours, definitely help your confidence.

"These are the matches that you live for, you practice for. You want to be on the centre court and play on such a high level for five hours.

"It's incredible."