US Open 2016: Former runners-up Caroline Wozniacki, Roberta Vinci advance to last 16

New York: Former runners-up Caroline Wozniacki and Roberta Vinci reached the US Open last-16 on Friday as two-time champions Novak Djokovic and Rafael Nadal prepared to push into the second week.

Wozniacki, who knocked out ninth-seeded former champion Svetlana Kuznetsova in the second round, clinched a seventh win in seven meetings over unorthodox Monica Niculescu of Romania.

The Dane`s 6-3, 6-1 victory on Arthur Ashe Stadium, which featured eight breaks of serve, was another noise-affected affair.

But instead of a din caused by chattering fans or rain on the roof, this time it concerned Wozniacki complaining that her opponent was making too much noise every time she employed her bizarre forehand slice.

The former world number one`s complaints helped extend the crucial ninth game of the opening set to 12 minutes.

"It`s fun but also frustrating," said 2009 and 2014 runner-up Wozniacki of her opponent`s unusual style.

"She makes you feel as if you can`t play tennis. You have to grind it out; it`s not pretty but you get a good workout as you have to run a lot."

Wozniacki, whose ranking has slipped to 74 after a long absence to fight an ankle injury, goes on to face either US eighth seed Madison Keys or Naomi Osaka of Japan for a quarter-final slot.

Vinci, the Italian seventh seed and runner-up to compatriot Flavia Pennetta in 2015, overcame a second set blip to defeat 21-year-old Carina Witthoeft 6-0, 5-7, 6-3.

The 33-year-old Vinci led 5-4, 30-0 in the second set before her 84th-ranked German opponent dug deep to take the tie into a decider.But Vinci prevailed on the back of 40 winners and will meet either Slovakia`s Dominika Cibulkova or Lesia Tsurenko of the Ukraine for a quarter-finl place.

"I got nervous when I served for the match, it was very dangerous," said the Italian who stunned Serena Williams in the semi-finals 12 months ago.

"But I just told myself `let`s start again, don`t think about the second set.`"

Latvia`s Anastasija Sevastova backed up her shock win over third seed and French Open champion Garbine Muguruza by making the last 16 for the first time with a 6-4, 6-1 win over Kateryna Bonaderenko of the Ukraine.

World number 48 Sevastova, who briefly retired from the sport in 2013, has matched her best run at a Slam -- a fourth round spot at the Australian Open in 2011.

Later Friday, Djokovic, well rested after not playing since Monday, resumes his title defence against fiery Russian Mikhail Youzhny, a two-time semi-finalist in New York.

World number one Djokovic claimed a laboured win over Poland`s Jerzy Janowicz in his opening match in which he required treatment on his right upper arm.

Fortunately for the two-time champion, he was able to rest up for the rest of the week after scheduled second round opponent Jiri Vesely handed him a walkover into the third round.

Youzhny, 34, and nicknamed "Colonel" by his fans, made the last-four in New York in 2006 and 2010 and is ranked at 61 in the world having been as high as eight in 2008.

Youzhny will start as the underdog against Djokovic, against whom he has a 3-6 losing record with his last win coming six years ago.

However, he did take a set off the Serb in the US Open quarter-finals in 2013.

Nadal, the 2010 and 2013 champion, takes a 3-0 lead over Andrey Kuznetsov of Russia into their night-time clash on Arthur Ashe Stadium.

German second seed and Australian Open champion Angelique Kerber closes the night session against American 17-year-old CiCi Bellis, the youngest player left in the women`s draw.

Victory for Kerber would take her to a season-leading 50 wins for 2016.

Elsewhere, the tussle for last-16 spots sees 2014 champion Marin Cilic facing America`s Jack Sock while American number one John Isner meets British outsider Kyle Edmund.