New York: World No. 1 Novak Djokovic received a free pass into the third round of the US Open when Jiri Vesely of the Czech Republic withdrew from their match on Wednesday due to a left forearm injury.

The walkover provided the top-seeded defending champion with an extended break to improve his niggling left wrist injury before his next match against either Guido Pella of Argentina or Mikhail Youzhny of Russia.

The 23-year-old Vesely, ranked 49th, beat Djokovic in the second round at the ATP Masters 1000 Monte Carlo event in April in their only meeting, making him one of two top 100 players with more wins than losses against the Serbian.

Seventh seed Roberta Vinci registered a speedy win over Christina McHale. Vinci, who was unseeded last year when she lost in the final to compatriot Flavia Pennetta, has offered early signs that a return trip could be in the offing as she raced into the third round with a 6-1, 6-3 win over American McHale.

The straight-forward victory, which Vinci wrapped up in 63 minutes, followed a clinical first round win over Germany's Anna-Lena Friedsam that took 68 minutes.

Two-time Wimbledon champion Petra Kvitova battled into the third round with a 7-6(2) 6-3 win over Turkey's Cagla Buyukakcay on Wednesday.

On Tuesday, World No. 1 Serena Williams opened her US Open account with a tidy 6-3, 6-3 win over Ekaterina Makarova, to join her sister Venus in the second round of the year's final Grand Slam.

Serena needed just 63 minutes to dispatch the 29th-ranked Russian. Germany's Angelique Kerber and Spain's Garbine Muguruza, who have both beaten Serena in slam finals this season, along with Poland's Agnieszka Radwanska, will all be eyeing the No. 1 ranking should the American's bid for a 23rd Grand Slam title stall.

Serena has owned the No. 1 spot since February 18, 2013 and will extend her streak to 186 consecutive weeks by the end of the US Open fortnight, tying the WTA record held by Steffi Graf.

Second seed Andy Murray has always said he would quite like an indoor Grand Slam event and at the US Open it seems he has found the next best thing.

The Scot cruised into the second round at Flushing Meadows with a 6-3, 6-2, 6-2 win over the Czech Lukas Rosol in the night session on Tuesday.

The new roof over Arthur Ashe Stadium was open but because of its size, Wimbledon and Olympic champion Murray said it made conditions almost identical to an indoor tournament.

"It's quite different playing out there now," Murray said. "There's literally no wind at all. It almost has the feel of playing indoors, it's perfect conditions. The roof, combined with the cooler evening air, made conditions easier to control the ball as well as ramping up the decibel levels for the traditionally noisy New York crowd.

Murray will play Marcel Granollers of Spain in the second round.

Fourth-seeded Agnieszka Radwanska of Poland, trying to make it to the quarter-finals in New York for the first time, breezed past US qualifier Jessica Pegula 6-1, 6-1 and fifth-seeded Romanian Simona Halep also eased through with a 6-0, 6-2 victory over Belgian Kirsten Flipkens.

Third seed Stan Wawrinka of Switzerland, a two-time semi-finalist, reached the second round with a 7-6 (7/4), 6-4, 6-4 win over Spain's Fernando Verdasco.

Kei Nishikori, who became Asia's first men's Grand Slam finalist in New York in 2014, when he fell to Marin Cilic in the final, also advanced, downing German Benjamin Becker 6-1, 6-1, 3-6, 6-3.

Eighth-seeded Dominic Thiem of Austria needed five sets to secure his second-round spot, downing Australia's John Millman 6-3, 2-6, 5-7, 6-4, 6-3.

Juan Martin del Potro, the 2009 US Open champion whose career was nearly ended by three wrist surgeries, advanced with a 6-4, 6-4, 7-6 (7/3) victory over fellow Argentine Diego Schwartzman.

There was a little record-setting on the men's side as well, with Croatian Ivo Karlovic belting a US Open record of 61 aces in a 4-6, 7-6 (7/4), 6-7 (4/7), 7-6 (7/5), 7-5 win over Taiwan's Lu Yen-Hsun.

Karlovic, 37, beat the previous best of 49 aces for one match in New York established by Richard Krajicek in 1999. "I knew there were a lot of aces because there was a period when almost every serve was an ace," Karlovic said.

Back on Arthur Ashe Stadium court for the first time since a shock loss to Italy's Roberta Vinci in last year's US Open semi-finals, Serena made quick work of Makarova to follow the lead of her sister Venus.

Sixth-seeded Venus had provided the opening act for sister Serena when she outlasted Ukraine's Kateryna Kozlova 6-2, 5-7, 6-4 before turning over the Arthur Ashe Stadium court to her top-seeded sibling.

<>The Flushing Meadows spotlight is once again firmly fixed on Serena as she chases an Open Era, record smashing 23rd Slam title with concerns over her right shoulder, believed to be a factor in her third-round singles loss at the Rio Olympics.

As always at the US Open, what the world No. 1 was wearing attracted as much attention as her performance. Serena sported a black tennis dress with cutouts revealing her muscular shoulders.

"It's a part of the design," explained Serena. "It's just the latest and greatest accessory.

"It also is functional, so I think that helps me, especially with my shoulder problems that I've been having."

The 34-year-old American took something off her big serve, but showed no signs of distress as she blasted 12 aces past Makarova, a player who on occasion has given her problems, beating her in the fourth round of the 2012 Australian Open.

"I didn't hit them as hard as I normally hit them," she said about her serves. "I just went for more placement. I didn't go for the big 120s (mph), just the regular.

"I knew today I needed to be focused," said Serena. "She goes deep in majors. She knows how to play big matches on big courts. She's not intimidated." As usual, history beckons Serena who is bidding for a seventh US. Open victory that would surpass the Open Era record she shares with Chris Evert. But whether it is the world number one ranking or another record at stake, Williams had the same answer. "I don't answer those questions," flatly stated Serena , who will next face American wild card Vania King, a 7-6(2) 6-3 winner over Germany qualifier Antonia Lottner.