MELBOURNE, Australia -- Matches were suspended at the Australian Open for two hours Monday because of extreme heat and humidity.

The temperature topped 95 degrees, and smoke from nearby wildfires drifted over Melbourne Park. The heat and humidity were high enough for tournament referee Peter Bellenger to suspend matches on outside courts.

Matches in progress had to be completed, and no new ones could begin. Top-ranked Serena Williams was on center court, leading 2-1 in the second set, when the announcement was made at 2:15 local time. She went on to defeat Eleni Daniilidou 6-4, 6-1.

Organizers had the option of closing the stadium roof for center court matches.

"It was hot, but it could be worse,'' Williams said. "I live in Florida, where you lose 20 pounds walking outside; this is nothing.''

Play on outside courses resumed at 4:15 p.m. in milder conditions.

The tournament's Extreme Heat Policy goes into effect when the temperature reaches 95 -- a reduction from 100 last year -- and when a heat stress measure known as the wet bulb globe temperature reaches 28. Both must occur simultaneously.

The wet bulb globe temperature is a combination of air temperature, humidity, intensity of solar radiation and wind speed.

Play on outside matches was expected to resume later Monday.

Players often have complained about the heat during the season-opening major, prompting the International Tennis Federation to consider pushing the tournament to a later starting date.