"I know how tough is this situation, I had the same a few years ago at this tournament," Nadal said. "Since the beginning, I saw a little bit he had some problems on the leg."
Federer started his record 57th consecutive Grand Slam tournament with a 6-4, 6-4, 6-2 win over Australian wild-card entry James Duckworth on a day when the heat forced at least one player to black out during his match.
Azarenka played the opening match on the center court, and said it felt "like you're dancing in a frying pan." She had a 7-6 (2), 6-2 win over No. 91-ranked Johanna Larsson of Sweden.
Wimbledon champion Murray, on the comeback from minor back surgery in September, had a 6-1, 6-1, 6-3 win over Go Soeda of Japan and No. 5 Juan Martin del Potro rallied for a 6-7 (1), 6-3, 6-4, 6-4 win over U.S. qualifier Rhyne Williams.
Former No. 1-ranked Lleyton Hewitt lost in five sets to No. 24-seeded Andreas Seppi of Italy only a week after beating Federer in the Brisbane International final.
A hot, gusty breeze swirled across Melbourne Park all day, making conditions more challenging instead of cooler. The crowd for the day session was 35,571, almost 12,000 down on day one.
Players draped bags of ice over their necks and shoulders and sat under covered seats in the changeovers. They retreated into the shade at the back of the courts between points.
Canadian qualifier Frank Dancevic said he blacked out during a 7-6 (12), 6-4, 6-3 loss to No. 27 Benoit Paire of France. Dancevic had treatment in the second set but continued.
"I think it's definitely hazardous to be out there," Dancevic said. "It's dangerous."
Murray agreed the conditions bordered on being dangerous.
"It's easy to say that the conditions are safe ... but it only takes one bad thing to happen and it looks terrible for the sport when people are collapsing," Murray said. "Whether it's safe or not I don't know, but you've got to be very careful."
Tournament referee Wayne McKewen defended a decision not to invoke measures which would have seen outdoor matches suspended and the roofs on two arenas closed.
"While conditions were hot and uncomfortable, the relatively low level of humidity ensured that conditions never deteriorated to a point where it was necessary to invoke the `Extreme Heat Policy,"' McKewen said.
No. 13 John Isner, the only seeded American man in the draw, retired with an injured right ankle after losing the first two sets against Martin Klizan. Czech veteran Radek Stepanek retired with a sore neck in the fourth set against Blaz Kavcic, but said it wasn't heat-related.
No. 10 Jo-Wilfried Tsonga, the 2008 Australian finalist, advanced with No. 11 Milos Raonic, No. 16 Kei Nishikori, No. 22 Grigor Dimitrov and No. 31 Fernando Verdasco.
In the longest match of the day, No. 18 Gilles Simon of France beat Daniel Brands of Germany 6-7 (4), 6-4, 3-6, 6-3, 16-14 in four hours, 32 minutes. Simon came into the tournament with an ankle injury sustained in a warmup tournament.
American Sloane Stephens, who beat Serena Williams in the quarterfinals here last year, defeated Yaroslava Shvedova 7-6 (1), 6-3 and 2004 U.S. Open champion Svetlana Kuznetsova and 19th seed was beaten 6-3, 6-3 by Elina Svitolina of Ukraine.
No. 5 Agnieszka Radwanzka, No. 8 Jelena Jankovic and No. 10 Caroline Wozniacki advanced.
