Mirjana Lucic-Baroni ousted the No. 2 player in the world with two straight aces.

"It was really easy," she said through tears later. "Right now it's hard."

Hard thinking back to the 15 years since she last made a Grand Slam round of 16. The days spent working out wondering if she'd find the financial backing to return to competition. The injuries that would crop up just as she was regaining her form.

She kept going, kept running herself breathless in practice, kept grinding through minor-league tournaments — all for a moment like Friday. At age 32, Lucic-Baroni stunned second-seeded Simona Halep 7-6 (6), 6-2 in the third round of the U.S. Open.

She was 17 when she reached the 1999 Wimbledon semifinals, the last time she got this far on the Grand Slam stage. Lucic-Baroni, who won the 1998 Australian Open doubles title with Martina Hingis, was a prodigy: At her very first WTA event as a 15-year-old, she won. That year, she made it to the third round at the U.S. Open.

Like many tennis aficionados, Lucic-Baroni watched 15-year-old CiCi Bellis play her second-round match Thursday. The Croatian veteran was charmed by the youngster's wide-eyed delight. She also couldn't help but contrast that to herself at the same age.

"I was feeling like an adult at 15 when I played here," Lucic-Baroni said. "She's just a little girl."

It's as though Lucic-Baroni is living her life in reverse — stoic as a teen, childlike as a 30-something.

"I was so young and I was so good and I was winning so much that even though it's exciting, it wasn't really a big deal," she said. "It was just a natural progression."

Now, she added, "Every round is amazing."

"In a way, I know I sound like and I feel like a little kid, like this is the first time ever happening," she said. "I love the feeling."

She didn't play a Grand Slam match for nearly eight years from the 2002 U.S. Open through 2010 Wimbledon.

"When I would get my chance on a big court against a big player, I wanted it so bad that I kind of was paralyzed," she said.

She'd fret: "OK, how many more do I have? I have to do it now."

Finally she relaxed and told herself: "Just play tennis."

But in Doha in February, her back gave out against Agnieszka Radwanska. At Indian Wells the next month, she had a bulging disk in her neck.

She was on an eight-match losing streak coming into U.S. Open qualifying and trailed 4-2 in the third set of her first match. Then in her last match, Lucic-Baroni was down 5-2 in the third, two points from defeat, with her opponent serving to clinch the spot in the main draw.

By comparison, facing three set points Friday against the French Open runner-up seems less daunting. With Halep trying to serve out the first set at 5-2, Lucic-Baroni broke back after five deuces. Halep started unraveling, and Lucic-Baroni started rolling.

"Everything was in for her," Halep said.

For years, coaches tried to change Lucic-Baroni's aggressive style. She said she's playing much better now that she's "starting to feel like myself again finally."

"I'm not the type of player that builds points and loops balls and kind of waits for their opportunities," she said with a laugh.

"I enjoy hitting winners," she added. "I enjoy hitting aces."

She had 31 winners Friday with 36 unforced errors. There was only one ace until the final two points of the match.

Lucic-Baroni had only once before defeated a top-five opponent — that happened during the 1999 Wimbledon run, against Monica Seles. She next faces 13th-seeded Sara Errani, who beat Venus Williams in three sets Friday.

"I feel fit. I feel strong in my mind," Lucic-Baroni said. "I feel very excited, even after so many years on tour."

She got married in November 2011 and said "I would be perfectly fine having a family."

"I don't need to do this, you know," she said. "But people don't realize how much I want this and how hard I worked for this."