* No more moaning about the heat for Serb
* Steams into third round match with Russian Tursunov (Writes through, adds byline)
By Ian Ransom
MELBOURNE, Jan 15 (Reuters) - The Novak Djokovic that bowed out of his first Australian Open title defence with heat sickness in 2009 has now been replaced by a mentally and physically tougher upgrade not prone to meltdowns, the world number two said.
After a rusty opening round, Djokovic's bid for a fourth successive title at Melbourne Park shifted up a gear on Wednesday as he trounced Argentine Leonardo Mayer 6-0 6-4 6-4 in Rod Laver Arena to march to the third round on another day of stifling heat.
Players have grumbled about the conditions, and a Canadian in the men's draw described the tournament's organisers as "inhumane" after he fainted on Tuesday when temperatures peaked at 42 degrees Celsius (108 Fahrenheit).
In 2009, a 21-year-old Djokovic was criticised by players and pundits for pulling out of his quarter-final when trailing American Andy Roddick on a day of extreme heat, having earned a reputation for retiring when the going got tough.
There was little moaning from the Serb on Wednesday, however, after he wrapped up his match against 98th-ranked Mayer in less than two hours to escape the 40-degree heat.
Apart from the hat-trick of titles won at Melbourne Park from 2011-13, Djokovic said there were "plenty of differences" between himself and the 2009 model.
"Obviously as the years go by, I'm more mature as a player, as a person. I learned new things in life. I developed myself," he said.
"I physically got stronger - mentally also. All of this plays, of course, an important role when you are playing in such conditions.
"It's not easy. Maybe it looks (easy), but I do go through tough times after long rallies, as everybody, trying to get some air, obviously.
"Generally it's much more efficient for me nowadays to recover and to get ready for the next point than it used to be in 2009 when I retired in my match against Roddick, yeah."
Djokovic completed his transformation into tennis's undisputable iron-man by outlasting super-fit Spaniard Rafa Nadal to win the 2012 Australian Open in a final that lasted a grand slam record of just under six hours.
The 26-year-old plays Russian Dmitry Tursunov in the third round on Friday, another day forecast to exceed 40 Celsius.
"Everything is fine," Djokovic repeated like a Buddhist invocation during his post-match media conference when asked about his ankle which rolled during a first-set tumble against Mayer.
"I'm physically fit. I've been practising, preparing for the Australian summer that we all know can be difficult at times with the conditions.
"Knowing that I played a day match today, I prepared myself mentally for that.
"It's not just physically. Mentally you need to be tough enough to not give up and not think about what conditions can do to you." (Editing by John O'Brien)