Wimbledon doubles champions Vasek Pospisil of Vancouver and Jack Sock of the United States both overcame injury to reach the second round of the U.S. Open Thursday with a 6-4, 6-4 defeat of Jarkko Nieminen and Henri Kontinen.

Pospisil had been bothered by a shoulder injury during a singles loss to Simone Bolelli this week while Sock had to quit in his first-round singles with a calf injury and cramping.

Pospisil didn't even know his partner was hurt until after Sock's singles loss.

"I didn't know he had to retire," he said. "I only ran into him after his match and he told me."

As for his shoulder, Pospisil said it's improving every day.

"The problem just came at a bad time," he said. "I'm serving OK but not great yet."

The eighth seeds both played without apparent problems as they completed their efficient victory in 70 minutes. The winners broke twice from 11 chances and never faced a break point from the Finnish Davis Cup team.

Pospisil and Sock lost their previous match, the Western & Southern Open final in Cincinnati, to Bob and Mike Bryan. That ended an ATP-record run of 14 straight wins as a new team, beginning with their start together at Wimbledon in June.

The pair, which remain first and foremost dedicated to singles, are also working to reach the ATP year-end finals in London next November as a doubles team.

"We are both singles players, we're fully focused on singles," Pospisil said. "But having a chance to reach London changes things.

"We're trying to keep relaxed on court, not stressing and enjoying it. We're not putting pressure on ourselves."

Later Thursday, men's No. 5 Milos Raonic of Thornhill, Ont., faced Germany's Peter Gojowczyk while women's seventh seed Eugenie Bouchard plays Romania's Sorana Cirstea.

Ivanovic out

Ana Ivanovic's comeback tour has ended with yet another early exit at the U.S. 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 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 U.S. 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.

Djokovic, Serena Williams move on

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 U.S. 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 U.S. 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."

In other action under the lights, 2012 U.S. Open champion Andy Murray won in straight sets.

Among the noteworthy results Thursday was the 7-6 (2), 4-6, 6-4, 6-2 win by Victor Estrella Burgos of the Dominican Republic against Borna Coric of Croatia. Playing in the U.S. Open for the first time at age 34, Estrella Burgos eliminated a player half his age in the 17-year-old Coric.

CiCi Bellis, the first 15-year-old since 1996 to win a U.S. Open match, could not follow that up with another victory, losing in the second round.

Bellis, a Californian who is ranked 1,208th and playing in her first tour-level event, won seven games in a row during one stretch but wound up getting beaten 6-3, 0-6, 6-2 by 48th-ranked Zarina Diyas of Kazakhstan.