Sydney NSW, Australia, 7 January 2018 | Matt Encarnacion (AAP)

Sydney’s heatwave claimed its first scalp in the opening match on the main court of the Sydney International, with fifth seed Kristina Mladenovic retiring during her match with Australian wildcard Ellen Perez.

With play starting an hour earlier than previously scheduled, the mercury soared to 39.1 degrees at 10.30am on Sunday, forcing tournament officials to activate their heat policy just half an hour into the opening day.

Players on the outside courts at Homebush were offered up to 15-minute breaks between the second and third sets, however it is believed no matches were suspended.

Temperatures hit a high of 44.5 degrees just after 1pm, and not long after Mladenovic pulled the plug on her match with the world No.343 Perez.

Mladenovic, who had been treated a handful of times by trainers during the match, was trailing 6-4 4-2 when she succumbed to the conditions on Ken Rosewall Arena.

The 22-year-old Perez, who was appearing in just her second ATP tournament, said she embraced the challenge of playing in stifling heat.

“It was definitely hot. I’m not going to lie. I do enjoy the heat and I definitely think it was getting to me a little bit but maybe not as much as she felt it, but it was hot out there,” Perez said.

“I didn’t notice at the start until she called the trainer out in the first set and obviously it was kind of prominent in the second set, so I knew I had to get a good start and keep it going.”

“I just tried to stay switched on. I knew I had to get a good start and get that lead and keep the pressure on. It’s a harder battle if you are losing and you don’t feel too well.

“So I knew I had to get that game or two up.”

Not only was it Perez’s first victory on tour, but her first encounter against a top-50 opponent.

“I’ve been playing college tennis so to get this opportunity was amazing. To get that first win helps my confidence and moving forward in the WTA, I feel really good about that,” she said.

“I played the US Open and got to play Shuai Zhang who was 20-ish around there, and got killed, so it was nice to get a win.”

Defending women’s champion Johanna Konta said players were used to the conditions, which is now required to be reviewed every half hour according to the activated regulations.

“It gets very hot here although last year I played here and I think it was my quarter-final match. It was very hot on that day as well,” Konta said.

“But in general as the tennis tour goes, we generally follow the sun, so we do encounter (heat in) a lot of different places.”