NEW YORK: World number ones Serena Williams and Novak Djokovic launched their US Open title bids in ruthless style on Monday, while Kei Nishikori's hopes of making another magical run to the final came to a shuddering halt.

Opening day at Flushing Meadows produced plenty of shock and awe as Djokovic delivered a jaw-dropping performance, speeding past Joao Souza of Brazil 6-1 6-1 6-1 as if the Serb had a taxi meter running outside the National Tennis Center.

Williams also played as if she had an appointment to keep as she began her final push toward a calendar year Grand Slam by pounding Vitalia Diatchenko 6-0 2-0 before the ailing Russian waved the white flag and retired injured.

Eighth seed Rafael Nadal, a two-time US Open champion, needed a bit more time to get the job done, the Spaniard taking nearly three hours to fight off a challenge from 18-year-old Croatian prospect Borna Coric 6-3 6-2 4-6 6-4.

Following a Broadway-style opening ceremony complete with orchestra and choir, Williams stepped onto center court under a spotlight to loud applause from an adoring New York crowd just seven matches from her quest to sweep this season's slams.

The six-time champion would exit Arthur Ashe Stadium just 30 minutes later to more applause and her target reduced to six more wins to join the exclusive club of calendar Grand Slam winners Maureen Connolly, Margaret Court and Steffi Graf.

"It's been really amazing, I can feel the support and the love on my journey for this milestone," said Williams, who won the first of her 21 majors at Flushing Meadows 16 years ago.

"I'm so ready. Whatever happens I'm here, I'm at home where it all began in 1999, so this is incredible for me to still be here and playing well.

"It's kind of awesome that this is the last grand slam of the year, because if it were in a different country, I think I would still love it but it's not the same as being an American playing in New York, playing for that ultimate goal."

NISHIKORI NIGHTMARE

Frenchman Benoit Paire provided the big shock of the day, grinding out a 6-4 3-6 4-6 7-6(6) 6-4 win over Nishikori, after the Japanese fourth seed squandered two match points in the fourth set tiebreak.

Marin Cilic, who beat Nishikori in last year's final, had no such opening day hiccups as the ninth-seeded Croat began the defense of his title with a 6-3 7-6(3) 7-6(3) win over Argentine qualifier Guido Pella.

"It's always very sad to lose in the first round, but I think he was playing good tennis," Nishikori lamented. "I don't think I played badly. Didn't play great, but still, it's never easy first match."

The 25-year-old had plenty of company at the departure lounge following a string of shocks that included Serbian seventh seed and former world number one Ana Ivanovic, eighth seeded Czech Karolina Pliskova and 10th-seed Carla Suarez Navarro of Spain.

A hot, muggy day got off to an ominous start when 50th-ranked Slovakian Dominika Cibulkova dispatched Ivanovic 6-3 3-6 6-3 on the Arthur Ashe Stadium court.

Anna Tatishvili gave the home fans a reason to get excited as the 121st-ranked American qualifier destroyed Pliskova 6-2 6-1 while Frenchwoman Oceane Dodin kept the upsets trending with 2-6 7-5 6-3 victory over Jelena Jankovic, the 21st seed and 2008 US Open runner-up.

Swiss teenager Belinda Bencic, one of just two players to beat Serena Williams this season, strolled past Bulgarian Sesil Karatantcheva to stay on track for a quarter-final showdown with the 33-year-old American.