Although players wilted and complained in similar conditions the previous two days, the tournament referee, Peter Bellenger, did not have the option of closing the roof because it would not have been fair to all the competitors: some would have played in the heat and some avoided it.
Before the quarter-finals, which yesterday saw Carlos Moya, Michael Chang, Mary Pierce and Amanda Coetzer all win, some singles matches are played on Centre Court, and others on outer courts with no roof. But starting with yesterday's games, all singles matches are on the 15,000-seat Centre Court and the roof can be closed at Bellenger's discretion if temperatures reach 35C.
Still, some players wanted the roof open so their fitness or playing style, better suited to wind and sun, might give them an edge. "I was going to play anyway so even if it's snowing, raining or 60 degrees I don't care," the unseeded Moya said after beating his fellow Spaniard Felix Mantilla in four sets.
"I think it may have been a different match in the sun and wind," a furious Mantilla said.
Bellenger said: "We haven't had 40-degree temperatures in Melbourne for eight years. I thought it was better for the game, for the players and for the public, to be able to view the game in relative comfort. I think in all this, the public deserves some consideration."
Moya had to put friendship aside as he disposed of the 14th-seeded Mantilla 7-5, 6-2, 6-7, 6-2. Once considered just another Spanish clay-court baseliner, Moya has grown in confidence since beating the defending champion, Boris Becker, in the first round. Yesterday he regularly came to the net. It was Spain's best performance at the Australian Open since Andres Gimeno lost the 1969 final to Rod Laver.
Moya now meets Chang, the world No 2, who reached the semi-finals for the third successive year. Chang, who has not won a Grand Slam event since the French Open eight years ago, ruthlessly disposed of the ninth-seeded Chilean Marcelo Rios 7-5, 6-1, 6-4 in just under two hours.
Chang has dropped only one set in his past five matches at Melbourne Park. He was a beaten finalist here last year and has also been runner- up at the French and US Open in the past two years. He said he did not feel under any pressure.
"I'm not too concerned about whether or not I'm able to win another Grand Slam title and stuff like that," he said. "I like to think things are getting better and better... I still feel like my best tennis is ahead of me."
In the first women's quarter-final, South Africa's Amanda Coetzer showed the same form which helped her bring down top seed Steffi Graf as she beat Kimberley Po of the United States 6-4, 6-1. Although Po managed to break Coetzer's serve once in each set, the South African converted six out of six break points.
Coetzer also criticised the decision to shut the roof, although she conceded it had given her and Po a break from the heat. "I definitely would have preferred playing outside. I love playing out in the sun," she said.
Pierce, in her quarter-final against Sabine Appelmans of Belgium, lost the first set 6-1 before levelling in the second and then came back from 3-0 down in the third to win 1-6, 6-4, 6-4.
"That's the great thing about tennis, you never know what's going to happen," she said, looking forward to a tough semi-final with Coetzer.
n Second-seeded Britons David Sherwood and James Trotman defeated Croat Ivan Ljubicic and Italian Federico Luzzi 6-3, 6-4 in the second round of the boys doubles.
P SAMPRAS (US) or A COSTA (Sp)
v G IVANISEVIC (Croa)
or T MUSTER (Aut)
M CHANG (US) v C Moya (Sp)
A COETZER (SA) v M Pierce (Fr)
M-J FERNANDEZ (US) or
D Van Roost (Bel)
v M HINGIS (Swit) or
I SPIRLEA (Rom)