Qualifier Mirjana Lucic-Baroni upset US Open second seed Simona Halep to reach the fourth round at a grand slam for the first time in 15 years.

The former child prodigy is now 32 but rolled back the years with a superb display of power hitting to knock out title contender Halep 7-6 (8/6) 6-2.

A title winner at 15 and Wimbledon semi-finalist by 17, Lucic-Baroni's burgeoning career was derailed by off-court problems.

In 1998, she and her mother and siblings fled their native Croatia for the United States to escape father and coach Marinko, who she alleges physically and verbally abused her from a young age.

Although she went on to have her great run at Wimbledon in 1999, beating Monica Seles before losing narrowly to Steffi Graf, Lucic-Baroni's success petered out.

Financial problems limited the amount she could play and she contested only two professional matches between 2004 and 2006.

But Lucic-Baroni was determined that her personal misfortune would not stop her playing tennis and in 2007 she began the long road back.

She has battled away on the lower rungs of the tennis ladder, returning to the grand slam stage in 2010 after a gap of eight years.

"I didn't get wildcards, I didn't get to just pick nice events and play," she said.

"I played qualifying of every not-awesome tournament there is everywhere in the world.

"I worked my way back and I earned it. Then when I got to grand slams, I wanted it so bad that when I would get my chance on a big court against a big player, I kind of was paralysed.

"I couldn't do it. It was always like, 'Okay, how many more do I have? I have to do it now. I have to do it now'. I finally relaxed and I said, 'Just play tennis'. It's the best day of my life."

Lucic-Baroni watched 15-year-old American Cici Bellis playing her second-round match on Thursday and was taken back to her own days as a teenage prodigy.

She said: "I remember it was really exciting, but back then it was so normal. 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. It was just a natural progression.

"And now it's just amazing. Every round is amazing. Every round I look forward to. I know I sound like and I feel like a little kid, like this is the first time it's ever happening. I love the feeling."

Halep had reached at least the quarter-finals at every other grand slam this season, losing to Maria Sharapova in the French Open final.

Maria Sharapova overcame Sabine Lisicki to set up a tasty fourth-round clash with Caroline Wozniacki. 

There was nothing pretty or straightforward about Sharapova's 6-2 6-4 victory, which took an hour and 41 minutes, but she was the tougher in the end under the lights on the Arthur Ashe Stadium court.

Lisicki had only won one of six previous meetings against Sharapova, at Wimbledon in 2012, but her big game means she is always a danger.

Last month the German hit a serve measured at 131mph, a world record for a woman.

Her radar was off in her first service game, though, with two double faults proving very costly as Sharapova ground her way into a 3-0 lead.

The Russian was coming under pressure in every service game but resisted impressively, and another Lisicki double fault eventually handed her the set.

Lisicki is known for her attacking style but she showed tremendous powers of defence to move 3-1 ahead in the second set.

However, Sharapova is the ultimate scrapper and she took advantage of a horrible drive volley from her opponent to level proceedings.

The Russian had the momentum now and set up a chance to serve for the match at 5-3 but back came Lisicki, who was certainly playing at a much higher level now.

The pair traded crunching forehands but in the end it was a forehand drilled just wide from Lisicki that handed Sharapova victory on her third match point.

The fifth seed said: "She was a really tough opponent, a very aggressive big server. I just tried to concentrate on my return. I wasn't serving as well as I wanted to.

"In the end it was a few points. I was happy I was able to break her."

Wozniacki has had an impressive summer and was very solid in a 6-3 6-2 victory over 18th seed Andrea Petkovic.

"I think every match I get into a better groove out there and I'm playing better," said the Dane. "It's nice to be through to the fourth round again.

"It's going to be another tough one, but I'm excited about it."

There was also disappointment for Venus Williams, who served for the match against Sara Errani before losing a bizarre contest 6-0 0-6 7-6 (7/5).

Remarkably, it is the second time Williams has lost a match at Flushing Meadows having lost the first set to love and won the second to love - she was also beaten by Kim Clijsters in 2009.

Errani put her finger to her lips after her win but insisted she was not unhappy with the crowd, saying: "I've never heard the crowd that strong. I was shaking.

"It was unbelievably good. I think I will remember forever that moment. I don't know why I did that. I was not angry. It was with the tension, with the adrenaline in my body."

Lucic-Baroni will play Errani in the fourth round.

It was a day of shocks, and another arrived on Court 17 where 17-year-old Swiss Belinda Bencic knocked out sixth seed Angelique Kerber 6-1 7-5.

Bencic, who works with Martina Hingis' mother Melanie Molitor, said: "I played a really good match right from the start, and then I had a little bit of a time-out in the second set.

"But I'm happy that I came back. Last year I played juniors here, and this year I'm in the fourth round. So it's incredible."

In her first visit to the last 16 at a grand slam, Bencic will play eighth seed Jelena Jankovic, who dispatched Sloane Stephens' conqueror Johanna Larsson for the loss of one game.

Peng Shuai followed up her victory over fourth seed Agnieszka Radwanska in round two by defeating 28th seed Roberta Vinci 6-4 6-3.