When the match started, the WBGT was 32.6 — still above the threshold that had been surpassed for the first time in the two-week tournament on Sunday night.
The official policy says the referee can suspend play if the ambient temperature exceeds 40C and the WBGT index reading exceeds 32.5. No matches were suspended in a scorching first week even when the ambient temperature went well above 40C, because organisers said the WBGT index remained below the threshold. That is, only one of the criteria for suspending matches was met, not all of them.
With that in mind, not everyone was convinced with Sunday's roof ruling being based purely on the WBGT index. While it was hot, the temperature didn't exceed 40C, prompting international tennis media to question why that criteria suddenly became irrelevant just because it was a final.
Aussie legend Pat Cash opposed the decision, saying it was biased towards Federer because it deprived Cilic of the conditions he needed to cause an upset.
US great Jim Courier said the muggy conditions would slow the ball down, likely favouring Federer.
"It can be very muggy in there and make things very heavy, slow the ball down, and if it slows things down at all, it probably helps Roger more than it does Marin," Courier said.
Cilic agreed that he was hard done by, slamming the decision to close the roof in his post-match press conference. Having prepared to play in hot conditions, he said it made a "huge difference" to the match.
The air conditioning was also turned on inside the stadium. Federer was no fan of the humidity during his semi-final against Hyeon Chung, asking the umpire for the air-conditioning to be turned on in Friday night's encounter.
He will have appreciated the artificial help as he surged to a dramatic win on Sunday.
Lleyton Hewitt said early in the third set the outside temperature was between 32C-35C, but inside it was a much more tolerable 27C.
The roof being closed is hardly the only reason Federer won — both players faced the same conditions — but if Cash, Courier and Cilic are to be heard, it may have given him a slight edge before the first serve of the match.
Not everyone was opposed to closing the roof, however. Despite being criticised earlier in the tournament for not closing the roof or suspending matches because of the heat, New York Times tennis reporter Ben Rothenberg said organisers had done the right thing this time around by using common sense rather than going explicitly by the rule book.
Lleyton Hewitt had a different opinion to Courier, saying before the match the roof closure would create "heavy" conditions favouring the big-serving Cilic.
"Totally different dynamics for both players to go out and play in this situation," Hewitt said in commentary for Channel Seven. "It will be heavier and more humid as well than the dry heat outside, and normally it is a heavier kind of play. So, it normally favours the big server."
The heat has been a pressing issue all tournament, particularly in the first week when temperatures climbed upwards of 43C.
Gael Monfils complained to the umpire he was going to collapse and said he was "dying on the court" during his clash against Novak Djokovic, which the Serb said was played in "brutal" conditions.
Alize Cornet also said she thought she was going to faint during her loss to Elise Mertens, and received medical attention while being rubbed with icepacks on the side of the court.
"It's not reasonable ... I felt like I was in an oven," Cornet said. "I thought, 'We're crazy to be here.'
"It's great for the spectator but for us we're at our limit. I think there's a question for the heat rules."
But still players were forced to carry on under the blazing sun — until Sunday night, that is, when Federer and Cilic were shown mercy.
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