Which Tennis Major presents players with the toughest weather to play in?
Title favourites Serena Williams and Novak Djokovic resume their march through the Australian Open in round two action today.
The duo are back-to-back on Rod Laver Arena, with Williams gunning for her sixth Australian Open crown and Djokovic his fifth.
After crushing Australian teen Ashleigh Barty in round one, Williams, who last won the Australian Open in 2010, will take on world No 104 Vesna Dolonc.
Fellow Serb, Djokovic, the defending champion, is up against Argentina's Leonardo Mayer.
Mayer, world ranked No98, showed good form in the opening round as he steam-rolled Spaniard Albert Montanes 6-1 6-3 6-1.
Australian world No 120 Casey Dellacqua was also a comfortable first-round winner and will look to repeat that form against Belgian Kirsten Flipkens, who made the fourth round at Melbourne Park last year.
Australia's top-ranked female, Samantha Stosur, will lead the charge in the night matches.
Over the hurdle of the first round, Stosur takes on the in-form Tsvetana Pironkova.
Ranked 57, the Bulgarian was a surprise winner of the Sydney International title in the lead-up to Melbourne Park.
The first qualifier in three years to win a WTA Tour title, it was the 26-year-old's maiden WTA tour title and left Stosur wary.
''To go through quallies and win a tournament as tough as what Sydney means she must be playing very well,'' Stosur said on Pironkova's recent form.
Local men's hope Matt Ebden will follow Stosur on Rod Laver Arena in the night matches.
Ebden ground out a five-set encounter with France's Nicolas Mahut and faces another tough contest against world No.28 Canadian Vasek Pospisil.
On Tuesday, Australia's best male hopes bowed out.
Lleyton Hewitt lost a five-set thriller to Italian Andreas Seppi while Bernard Tomic retired with a leg injury after losing the first set of his clash with world No.1 Rafael Nadal 6-4.
FEELING THE HEAT
Australia's summer heat wave has produced eye-popping, knee-buckling temperatures, and the mercury soared to 42 degrees Celsius on day two of the Grand Slam tournament in Melbourne.
Health warnings have been issued and emergency services are bracing for fire danger as the heatwave sweeps across the country.
Officials in the state of Victoria were preparing for the most extreme weather since the Black Saturday bushfires of 2009, which killed 173 people and destroyed thousands of homes. The heatwave that came before the fires was responsible for 374 deaths.
While the conditions were remarkable, so was the ability of the world's top tennis players to endure them at the Australian Open in Melbourne, undercutting their reputation as coddled complainers.
It's nothing out of the ordinary with touring pros forced to cope with the frustration of frequent rain delays at Wimbledon, cool, damp weather at the French Open and changeable weather of both summer and autumn's ilk during the US Open.
But the heat served up at the Australian Open poses perhaps the most challenging conditions on the Grand Slam circuit.
''It can become just a very mental thing, you know, and you just can't accept that it's hot,'' Roger Federer said.
''Just deal with it, because it's the same for both. That's basically it.''
Carolina Wozniacki of Denmark said the plastic on her water bottle began to melt when she set it down on the court during her match. But she said an ice bath helped her recover quickly afterward.
''I could go out and play another two sets now,'' Wozniacki said.
''You know that it's going to be hot. It's Australia.''
With more 40C temperatures in the forecast for Melbourne, here are some things to know while the heat is on:
GUIDELINES FOR HALTING PLAY
Matches can be stopped and the tournament's two retractable roofs closed at the discretion of tournament director Wayne McKewen. He makes his decision while monitoring the wet bulb globe temperature index, or WBGT, a calibration based on temperature, humidity, wind speed and sunshine.
Extreme heat halted plays during several days of the 2006 tournament. The hottest Australian Open on record was in 2009, when the average temperature was 34.7C.
McKewen described the weather Tuesday as ''uncomfortable'' but said he allowed play to continue because the humidity was low. American John Isner said the heat was the sort he feels when he opens his oven.
One ball girl was treated for heat stress, and the crowd for the day session was 35,571, almost 12,000 less than on Monday.
But while six players dropped out of the tournament Tuesday, none of the departures was linked to the weather. Gilles Simon of France and Daniel Brands of Germany made it through a match lasting four hours, including a 16-14 final set.
SOME DON'T LIKE IT HOT
Canadian qualifier Frank Dancevic said he blacked out during a three-set loss and described the conditions as dangerous. Reigning Wimbledon champion Andy Murray won easily but questioned whether matches should have been played.
''In this heat, that's when you're really pushing it to your limits,'' Murray said.
''You don't want to see anything bad happen to anyone.''
SOME CAN TAKE THE HEAT
Victoria Azarenka described her match as ''like you're dancing in a frying pan,'' but when she won, she decided she needed to sweat a little more. So she headed for a practice court.
''I just went out straight to go hit - actually put on a long-sleeved shirt,'' Azarenka said.
''It wasn't probably the smartest thing to do, but I'm fine.''
The Belarusian said she'll back out in the heat Wednesday, even though she has the day off.
''Maybe I won't hit for two hours,'' she said, ''but I'm definitely going to go out and, you know, get some more suntan.''