The Australian Open shouldn't be held in January unless changes are made to Melbourne Park to protect players and fans from the searing heat, a health and safety expert has warned.

Players and spectators sweltered through 39 degree conditions at Melbourne Park, with 42 degrees the forecast for Friday.

Some clever kids took refuge in the water fountains at Garden Square to beat the heat. Outside courts with shade had spectators sitting out of the sun while courts with no shade were empty.

The crowd of 38,072 was down about 10,000 people on the two previous day-time totals.

Most players looked uncomfortable, but Frenchman Gael Monfils battled the most. He lost to Novak Djokovic on Rod Laver Arena where the on court temperature was 69 degrees. He won the first set but then crumbled, virtually giving up in some points, to lose in four sets.