World No. 1 Novak Djokovic and Canadian star Milos Raonic advanced to the second round after a relatively easy opening day matchups at the U.S. Open on Monday.

Wimbledon champion Djokovic came off to a fast start, breaking Diego Schwartzman of Argentina six times to claim the first two sets at the Arthur Ashe Stadium in New York.

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The 27-year-old Serb made another break against the Argentinean on the third set to advance into the second round, 6-1, 6-2, 6-4.

Djokovic proved to be on the offensive as he launched seven service aces and converted 24 winners during his main event match, which finished just before midnight.

The seven-time Grand Slam champion Djokovic hasn't been playing good tennis after his win at Roland Garros. He exited early in the third-round in both the Rogers Cup and recently concluded Western and Southern Open.

"Toronto and Cincinnati were a bit tough emotionally for me," Djokovic said in an interview after the match. "But now it's different. I feel emotionally charged and ready to go."

The Serbian will now face Frenchman Paul-Henri Mathieu in the next round.

Meanwhile, the fifth-seeded Raonic won his match against Japanese Taro Daniel. He cruised by the first two sets, but stumbled on the third set as Daniel forced him into a tiebreak, which Raonic eventually won, 6-3, 6-2, 7(7)-6(1).

"I'd go in and out sometimes," Raonic said after the match. "Maybe just lacked a little bit of intensity."

The hard-hitting Raonic converted 20 service aces and an unprecedented 58 winners to close the match in just a little under two hours.

Raonic will now play German Peter Gojowczyk in the second round.

With defending champion Rafael Nadal not competing in the tournament because of a wrist injury, Grand Slam winner Roger Federer will be replacing him as second seed. He will see action on Wednesday's main event at 7:00 p.m. against Australian Marinko Matosevic.

TagsNovak Djokovic, Milos Raonic, rafael nadal, Roger Federer