Japan's Yuichi Sugita scored one of the upsets of the Australian Open's first round on Monday, beating eighth seed Jack Sock of the United States in four sets.

The 41st-ranked Sugita took 2 hours, 33 minutes to complete the 6-1, 7-6(4), 5-7, 6-3 victory over world No. 9 Sock, who appeared to be hampered by the hip injury he suffered in the previous meeting between the two players on Jan. 2 at the Hopman Cup.

The 25-year-old American retired from that encounter after hurting his hip in a fall, and was clearly not back to full health for Monday's match.

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The victory was nevertheless cause for celebration for Sugita, 29, who will face world No. 89 Ivo Karlovic of Croatia in the second round.

"To beat a top 10 player at a Grand Slam is a big step up for me, overcoming a hurdle, and I'm very happy," Sugita said.

"I fought hard in the rallies so as not to allow my opponent to play good tennis."

Yoshihito Nishioka earlier moved into the second round by beating world No. 29 German Philipp Kohlschreiber in five sets.

Nishioka, who underwent surgery last March to repair a torn knee ligament, won 6-3, 2-6, 6-0, 1-6, 6-2 in 2 hours, 39 minutes, despite Kohlschreiber scoring more aces and winners throughout the match.

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"Before I got injured I had played good tennis and beaten top opponents, and that experience was big for me," said Nishioka, currently ranked 168th in the world.

"Because the wind was strong, I didn't want to take risks, so I chose tactics that would reduce mistakes."

The 22-year-old reached the second round of the tournament for the second straight year, and will challenge Italian Andreas Seppi on Wednesday.

In the women's draw, Croatian Jana Fett proved too strong for Japan's Misa Eguchi, winning 6-3, 6-2 in 1 hour, 12 minutes.

"I didn't play well from start to finish," said Eguchi. "The strong winds played havoc with the toss on my serve and never let up."