Ana Ivanovic of Serbia serves to Karolina Pliskova of the Czech Republic during their match at the 2014 US Open tennis tournament in New York, Aug 28, 2014. [Photo/Agencies]

NEW YORK?- Ana Ivanovic's comeback tour has ended with yet another early exit at the US Open.

Ivanovic, who had been having her best season since winning the French Open in 2008, looked out of sync and had 29 unforced errors in a 7-5, 6-4, second-round loss Thursday to 42nd-ranked Czech Karolina Pliskova.

"It was just a really bad day," the eighth-seeded Ivanovic said. "My rhythm was really off. And, yeah, my forehand wasn't working at all."

Ivanovic had won 47 matches coming into Flushing Meadows, the most of any woman on the tour this year and she returned to the top 10 for the first time in five years. Her three titles equaled the career high she set in 2008, when she reached No 1.

But she hasn't matched that success in the Grand Slam tournaments. She made the quarterfinals of the Australian Open, but lost early at Roland Garros and Wimbledon. In 10 US Open appearances, she has made it as far as the quarterfinals just once.

"This is exactly what I think I have to reassess," the 26-year-old Serb said. "I had great lead-ups to every Grand Slam. I played a lot of matches and won lot of matches. ... At the Grand Slams I just haven't performed that well."

Ivanovic joins No 4 Agnieszka Radwanska as the highest seeds to fall in the tournament, which remained largely true to form on a sunny, blustery day of blowouts.

Top-seeded Novak Djokovic and Serena Williams both won in straight sets. Others through easily included No 9 Jo-Willfried Tsonga, No 10 Kei Nishikori and No 13 John Isner, who advanced to another third-round matchup with 22nd-seeded Philipp Kohlschreiber, the man who has beaten him the same round the past two years.

Wimbledon champion Petra Kvitova, the third seed, won in straight sets, as did 11th-seeded Flavia Pennetta.

Djokovic danced for the fans, doing a sort of salsa to Iggy Azalea's summer hit "Fancy," after his 6-1, 6-3, 6-0, rout of Frenchman Paul-Henri Mathieu.

Djokovic next faces American Sam Querrey, who downed 28th-seeded Spaniard Guillermo Garcia-Lopez in straight sets

Williams, who complained of swirling winds at Arthur Ashe Stadium in her second-round match against Vania King, double-faulted three times in her first game before getting it together for a 6-1, 6-0 victory.

That extended her US Open winning streak to 16 matches. She is trying to become the first woman to win the tournament three consecutive years since Chris Evert had a four-year run from 1975-78.

No 16-seeded Victoria Azarenka, who lost to Williams in the US Open finals the past two years, dropped the first three games before coming back to beat 44th-ranked American Christina McHale 6-3, 6-2 in another second-round match.

Azarenka, a two-time Australian Open champion who once held the No 1 ranking, has been hampered by injuries, including one to her left foot that kept her out for much of the season and a knee problem that led her to pull out of the tuneup event in Cincinnati.

"I don't want to talk about frustration," Azarenka said. "I just want to talk about positives."

Other anticipated matches Thursday included the return of 15-year-old American CiCi Bellis, two days after becoming the youngest player to win a match in the tournament since 1996. Bellis, ranked 1,208th and playing in her first tour-level event, faces 48th-ranked Zarina Diyas of Kazakhstan.