Title: List of Australian Open men's doubles champions
Content:  This is a comprehensive list of all the men's doubles finals with the resulting champions for the Australian Open tennis tournament.
Title: List of Australian Open singles finalists during the Open Era
Content:  The Australian Open is a Grand Slam tennis tournament held annually in Melbourne, Australia at the Melbourne & Olympic Parks grounds. Since 1969, the tournament became open to professionals, so it is now called the Australian Open. The senior men's and women's tournaments are open to any player with a world ranking, although players below number 100 in the world rankings generally have to enter a preliminary qualification tournament or receive a wildcard to gain entry. The men who have reached the final at least four times in the open era are Mats Wilander, Ivan Lendl, Stefan Edberg, Andre Agassi, Roger Federer, Novak Djokovic, Andy Murray, and Rafael Nadal. Wilander reached the final four times, three times while 
Title: List of Australian Open men's singles champions
Content:  Christchurch and Hastings, New Zealand, and Perth, Brisbane, Adelaide, Sydney and Melbourne, Australia, have hosted the men's singles event. The event switched cities every year before it settled in 1972 in Melbourne. The event was held at the Kooyong Stadium before moving to Melbourne Park in 1988. The Australian Open court surface changed once, from grass courts to hardcourts in 1988. Mats Wilander was the only player to win the event on both surfaces; twice on grass and once on hardcourt. The men's singles rules have undergone several changes since the first edition. This event has always been contested in a knockout format, and all matches have been best-of-five 
Title: 2019 Australian Open
Content:  The 2019 Australian Open was a Grand Slam tennis tournament that took place at Melbourne Park from 14 to 27 January 2019. It was the 107th edition of the Australian Open, the 51st in the Open Era, and the first Grand Slam of the year. The tournament consisted of events for professional players in singles, doubles and mixed doubles. Junior and wheelchair players competed in singles and doubles tournaments. The 2019 Australian Open was the first Australian Open to feature final set tie-breaks. Roger Federer and Caroline Wozniacki were the defending men's and women's singles champions, but were unsuccessful in their respective title defenses; Federer lost to Stefanos Tsitsipas in the fourth round and Wozniacki lost to Maria Sharapova in the third round. Novak Djokovic of Serbia won the men's singles title at the 2019 Australian Open, defeating Rafael Nadal of Spain in straight sets in the men's final. Naomi Osaka of Japan defeated Petra Kvitová of the Czech Republic in three sets to win the women's singles title. The tournament had a record attendance of 796,435 spectators.
Title: List of Australian Open singles finalists during the Open Era
Content: = Champion During the 53 times that this tournament has been held in the open era, 54 men have reached the Australian Open men's singles final. The final has included men from 19 different nationalities. Twelve of the 54 men have been from the United States, and eleven have been from Australia. Other countries well represented include Switzerland, Sweden, Spain, Russia, the United Kingdom, South Africa, Germany, France, Czechoslovakia, and Chile.
Title: 2017 Australian Open Series
Content:  Novak Djokovic was the two-time defending champion, but lost in the second round to the 117th ranked Denis Istomin from Uzbekistan. Roger Federer won his fifth Australian Open title, and 18th Major title overall, defeating Rafael Nadal in the final in five sets. With the win, Federer became the first male player to win at least five titles at three Grand Slam tournaments (five at the Australian Open, seven at Wimbledon and five at the US Open). This was the first Grand Slam tournament in which Andy Murray started as World No. 1 and top seed. Murray retained the top ranking despite losing to Mischa Zverev in the fourth round.
Title: 2011 Australian Open – Men's Singles
Content:  Novak Djokovic defeated Andy Murray in the final, 6–4, 6–2, 6–3 to win the Men's Singles tennis title at the 2011 Australian Open. It was his second Australian Open title and second major title overall. Roger Federer was the defending champion, but lost in the semifinals to Djokovic. With his quarterfinal win over Stanislas Wawrinka, Federer broke Jack Crawford's record for the most match wins at the Australian Open. Rafael Nadal, the 2009 champion, had the chance to become the first man since Rod Laver in 1969 to hold all four Grand Slam tournaments at once, having won the French Open, Wimbledon and US Open in 2010, but lost in the quarterfinals to David Ferrer. This was the last Australian Open appearance for 2001 finalist and former top 10 player Arnaud Clément.
Title: List of Australian Open men's singles champions
Content:  except in 1970, 1973, and 1974, when the first round was best-of-three sets, and in 1982, when the third and fourth round were best-of-three sets. Since 1905, all sets have been decided in the advantage format. The lingering death best-of-twelve points tie-break was introduced in 1971 and has been used for the first four sets since then, except from 1980 to 1982, when the tie-break was also played in fifth sets. The champion receives a miniature replica of the silver-gilt Norman Brookes Challenge Cup, named after the 1911 champion and former Lawn Tennis Association of Australia (LTAA) president, and modeled after the Warwick Vase. In the Australasian Championships, James 
Title: 2018 Australian Open – Men's Singles
Content:  Defending champion Roger Federer defeated Marin Čilić in the final, 6–2, 6–7(5–7), 6–3, 3–6, 6–1 to win the men's singles tennis title at the 2018 Australian Open. It was his record-equalling sixth Australian Open title (tied with Roy Emerson and Novak Djokovic) and record-extending 20th major title overall. With the win, Federer became the oldest man to win a major singles title since Ken Rosewall in 1972. This was the 10th time that Federer defended a major title, with the previous time being at the 2008 US Open. Čilić became the first Croatian to reach an Australian Open singles final. Federer and Rafael Nadal were in contention for the world No. 1 singles ranking. Nadal retained the top ranking by reaching the fourth round. It was the first time since the 2011 Australian 
Title: List of Australian Open men's singles champions
Content:  holds the records for most titles with three (1922, 1924–1925), and the most consecutive titles with two (1924–1925). In the Australian Championships, Roy Emerson holds the records for most titles with six (1961, 1963–1967) and most consecutive titles with five (1963–1967). The inclusion of professional tennis players in 1969 marked the competition's entry into the Open Era, in which Novak Djokovic (2008, 2011–2013, 2015–2016, 2019–2021) holds the record for most titles with nine. The Open Era record for most consecutive titles is three by Djokovic (2011–2013 and 2019–2021). This event was won without losing a set during the Open Era by Rosewall in 1971 and Federer in 2007.

Question: who has won the mens singles title in the australian open tennis tournament?
Answer: Roger Federer.