She achieved victory despite reeling when she served for the match at 5-4 in the second set. She blew a 30-0 lead with a tight right hand that landed just wide, a double fault, a mediocre backhand approach shot that Mertens slapped for a right winger, and then, with the tension rising, another double guilt.
Mertens bared his teeth and looked at his support group in the players' box, including his coach and boyfriend, Robbe Ceyssens. She and they knew very well that such an abrupt change of momentum could have great consequences. Wozniacki knew it too.
In 2011, after losing in three sets against Li, he observed grimly in his postgame press conference that "sometimes in games or in tennis it's a ball that can change everything".
Wozniacki, the best player Denmark has produced, who was No. 1 for 67 weeks in 2010 and 2011, recalled returning to the practice court in Melbourne after that loss to Li.
"I never do that after getting away in a tournament and losing after a tough battle," he said. "Usually, I take a day off the next day, I went back and practiced, I just needed to get some frustration, then I started winning and I won several tournaments in a row, at the same time, it was still a game that still hurt."
That party was still hovering on Thursday when she tried to close it. He had also saved two match points in his second-round match here against Jana Fett, coming back from 1-5 in the third set.
"It means a lot to me," he said after Thursday's victory. "I got very tight in 5-4, double fouls in pairs, and I thought that after two good first serves, I thought it was all calm, and everything went well, and then it was not good anymore."
He said that his 2011 defeat was "definitely on my mind" and that his normally strong legs suddenly began to tremble. Eventually he had to save two set points on his serve at 5-6 before taking control of his nerves and the match in the tiebreaker.
"I took some deep breaths," he said.
Now, after avoiding another semifinal nightmare in Melbourne, he will have another chance to fill another long-standing breach in his resume by once again trying to win his first Grand Slam. Title. Wozniacki reached two finals at the US Open: lost in 2009 against Clijsters and in 2014 against Serena Williams.
Clijsters and Williams are the type of power players that have hampered the most important titles for Wozniacki, a remarkable defender and the engine that for a long time lacked a truly decisive shot. But only the counterpunches, with styles similar to those of Wozniacki, are still in dispute at the Australian Open this year.
In Saturday's final, Wozniacki will face the winner of the other semifinal on Thursday, between number 1 Simona Halep and former No. 1 Angelique Kerber.
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