With the thermometer pushing towards 100 Fahrenheit (38 Celsius) the pair battled for two hours and 18 minutes under a ferocious sun before the 10th seeded Latvian ended the marathon on her fourth match point, sending the few remaining spectators at the Louis Armstrong Stadium court scrambling to find shade.
"Today (it) was a very tough match. She (Petkovic) was fighting for every point until the end," said Ostapenko, who will meet American Taylor Townsend in the second round. "I mean, of course conditions were difficult today.
"It was very humid and also very hot, but it was the same for both of us. I mean, we had to deal with it."
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Petkovic, having failed to qualify in Cincinnati and New Haven ahead of the U.S. Open, arrived at Flushing Meadows well rested and the 30-year-old needed all her reserves on a sweltering day that is sure to test the fitness of every player across the sprawling Billie Jean King National Tennis Center.
Ostapenko, the 2017 French Open champion, may have also benefited from an uninspiring build-up to the season's final grand slam, crashing out in the first round at Montreal and Cincinnati to leave her fresh going into her opener.
Still, the conditions were so oppressive that officials implemented the extreme weather rule, sending the players off the court for 10 minutes to get out of the heat.
"I think it helped me a lot," said Ostapenko. "I went, actually, to the bathroom and changed the outfit for the new one to be more fresh and just stayed in air conditioning.
"I think it helped me, because then I went back on court and I broke her serve straightaway.
As the score indicates, there was little to choose between the two players, Ostapenko hitting 97 winners to Petkovic's 94.
"It's very important to win those tough matches because first rounds are always tricky," said Ostapenko. "It's very important to win the match when you're not playing your best probably. It gives you more confidence."
(Editing by Ken Ferris)
