New York, Sep 1 (IANS): The US Open kicked off here with easy wins for World No.1 Novak Djokovic and defending champion Marin Cilic.

However, last year's finalist, the men's fourth seeded Kei Nishikori, on Monday suffered a shock exit as did seventh-seeded Ana Ivanovic, who lost her first round match, reports Efe.

Djokovic asserted his status as the clear favourite, as Brazilian Joa Souza was sent packing in only 71 minutes, with the scoreline reading 6-1, 6-1, 6-1.

The Serb played a flawless game, smashing eight aces and hitting 24 winners, while committing only seven unforced errors and no double faults.

He now awaits the winner of the match between Vasek Pospisil and Andreas Haider-Maurer, in the second round.

Cilic, the surprise winner last year, also sailed through to the next round with a 6-3, 7-6 (3), 7-6 victory over Argentine qualifier Guido Pella.

The ninth-seeded Croat will now face Russian Evgeny Donskoy, who got the better of Frenchman Lucas Pouille 6-2, 6-7 (3), 6-4.

Japanese Nishikori bowed out of the US Open, the tournament's first big upset this year, as he succumbed to the big-serving Frenchman Benoit Paire 4-6, 6-3, 4-6, 7-6 (6), 4-6.

Paire fired 21 aces and committed five double faults, compared to only three aces for the last year's finalist, who despite all efforts only managed to break the Frenchman's serve twice.

Paire blasted 64 winners with 67 unforced errors to Nishikori's 34 and 36 respectively.

The opening day of the year's last grand slam also witnessed Spain's David Ferrer, Canada's Milos Raonic, Belgium's David Goffin, France's Gael Monfils and Jo-Wilfried Tsonga, and Bulgaria's Grigor Dimitrov advancing to the second round.

The women's draw too had its share of upsets as seventh-seeded Ivanovic was outplayed by Slovakia's Dominika Cibulkova 3-6, 6-3, 3-6.

Meanwhile, CoCo Vandeweghe ousted fellow American Sloane Stephens 6-4, 6-3.

Other seeded players seen in action, Czech player Karolina Pliskova (8) and Venus Williams (23) also went past the first round.

Williams, who could possibly face her sister and World No.1 Serena in the quarter-finals, defeated Puerto Rican Monica Puig 6-4, 6-7 (7), 6-3.