MELBOURNE, Australia — Novak Djokovic opened his Australian Open title defense with a straight-sets win, before going 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 on Monday, 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 – and responded to allegations that in that last decade 16 players have been repeatedly flagged to the integrity unit. None of the players were named, although reports said all had been in the top 50. 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 said he thought the allegations 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. 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 … 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.”

Roger Federer started his bid for a fifth Australian title with a 6-2, 6-1, 6-2 win over Nikoloz Basilashvili in a night match, following Williams and Djokovic on the center court.

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.

“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,” said Williams, who dropped only one service game against Giorgi. “I was able to stay in it and stay calm today and I think that’s what matters most.”

The 21-time major winner said there was no hint of match-fixing on the women’s tour.

“I play very hard, and every player I play seems to play hard,” she said. “As an athlete, I do everything I can to be not only great, but, you know, historic.”

Maria Sharapova, who lost to Williams in the final here last year and could meet her again in the quarterfinals, showed no signs of a left forearm injury that curtailed her preparations when she beat Nao Hibino 6-1, 6-3 in a night match. Genie Bouchard advanced with a 6-3, 6-4 win against Alexandra Krunic.

Two-time Wimbledon champion Petra Kvitova beat Thai qualifier Luksika Kumkhum, avenging her upset loss in the first round here in 2014, and No. 4 Agnieszka Radwanska beat Christina McHale 6-2, 6-3. Also advancing were 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, 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, a semifinalist in 2013, lost 6-3, 6-3 to Chinese qualifier Wang Qiang, and exited along with No. 26 Anastasia Pavlyuchenkova and No. 27 Anna Karolina Schmiedlova.

On the men’s side, sixth-seeded Tomas Berdych, a semifinalist last year, opened his 50th consecutive major by beating Yuki Bhambri 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, No. 26 Guillermo Garcia-Lopez and No. 27 Grigor Dimitrov. No. 22 Ivo Karlovic retired due to a left knee injury while trailing Federico Delbonis 7-6 (4), 6-4, 2-1.