MELBOURNE, Australia

Novak Djokovic opened his Australian Open title defense with a straight-sets win, then had to go to the defense of his sport as reports of match-fixing overshadowed the first day of the season-opening Grand Slam.

Djokovic had a 6-3, 6-2, 6-4 win over Chung Hyeon of South Korea at Rod Laver Arena today, immediately after women’s defending champion and No. 1-ranked Serena Williams started her bid for a seventh Australian title with a 6-4, 7-5 win over No. 34-ranked Camila Giorgi.

Williams, who hadn’t completed a competitive match since her bid for a calendar-year Grand Slam in 2015 ended in semifinal defeat at the U.S. Open, was on court warming up when tennis authorities held a joint news conference at Melbourne Park to refute reports from the BBC and Buzzfeed News that match-fixing had gone unchecked in tennis.

ATP chairman Chris Kermode and Nigel Willerton, head of the Tennis Integrity Unit, represented the four governing bodies of tennis — the ATP, the WTA, the Grand Slam Board and the International Tennis Federation — in a hastily convened news conference to respond to allegations that in that last decade 16 players — all at some stage ranked in the top 50 — have been repeatedly flagged to the integrity unit.

Kermode said tennis authorities “absolutely reject any suggestion that evidence of match-fixing has been suppressed for any reason, or isn’t being investigated.”

Djokovic played his match, and then faced a grilling over the reports.

The allegations, he said, related to matches from almost 10 years ago and didn’t involve active players.

“I don’t think the shadow is cast over our sport,” he said. “People are talking about names, guessing who these players are, guessing those names. But there’s no real proof or evidence yet of any active players. As long as it’s like that, it’s just speculation.

“From my knowledge and information about, you know, the match-fixing or anything similar, there is nothing happening on the top level.”

He said he couldn’t give a definitive defense of lower-tier tournaments, but added “there is an organization, authorities, people who take care of that on a daily basis and make sure to track it down.”

Williams, like Djokovic, won three of the four major titles last season, but hadn’t finished a competitive match for months. She withdrew from the Hopman Cup because of inflammation in her knee, playing just one set in Perth.

Yet she dropped just one service game against No. 34- ranked Giorgi, who struggled with 12 double-faults.

“I haven’t played in a long time, but I have been playing for 30 years, so it’s kind of — I try to focus on that,” she said. “I was able to stay in it and stay calm today and I think that’s what matters most.”

Two-time Wimbledon champion Petra Kvitova earned the distinction of winning the first completed match at the tournament when she beat Thai qualifier Luksika Kumkhum, avenging her upset three-set loss in the first round here in 2014.

“Now I can breathe a little bit normally,” she said. “I got really nervous before the match. I didn’t sleep well actually. The memories from two years ago were a little bit difficult.”

Also advancing were No. 4 Agnieszka Radwanska, who beat Christina McHale 6-2, 6-3, No. 10 Carla Suarez Navarro, No. 12 Belinda Bencic, No. 13. Robert Vinci and No. 23 Svetlana Kuznetsova.

Former No.1-ranked Caroline Wozniacki lost 1-6, 7-6 (3), 6- 4 to Yulia Putintseva of Kazakhstan, continuing a downward spiral at Melbourne Park that has resulted in her exiting one round earlier each year since she reached the 2011 semifinals.

No. 24 Sloane Stephens, who beat Serena Williams in the 2013 Australian Open quarterfinals, lost 6-3, 6-3 to Chinese qualifier Wang Qiang, while No. 26 Anastasia Pavlyuchenkova and No. 27 Anna Karolina Schmiedlova also went out in the first round.

On the men’s side, sixth-seeded Tomas Berdych, a semifinalist last year, opened his 50th consecutive major by beating Yuki Bhambri of India 7-5, 6-1, 6-2, and No. 7 Kei Nishikori had a 6-4, 6-3, 6-3 win over Philipp Kohlschreiber.

Also advancing were No. 12 Marin Cilic, No. 14 Gilles Simon, No. 15 David Goffin, No. 19. Dominic Thiem and No. 26 Guillermo Garcia-Lopez. No. 22 Ivo Karlovic was the first of the men’s seeds to exit the tournament when he retired due to a left knee injury while trailing Federico Delbonis 7-6 (4), 6-4, 2-1.

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