Novak Djokovic defended his Australian Open crown and won a 17th Grand Slam after digging deep into his physical and mental reserves to beat Dominic Thiem in a gripping five-set final.The Serbian second seed won 6-4 4-6 2-6 6-3 6-4 for a record-extending eighth title at Melbourne Park.Djokovic, 32, twice called the doctor while trailing before fighting back.Austrian fifth seed Thiem, 26, still awaits his first major title after losing a third Grand Slam final.Djokovic, who fluctuated from steely brilliance to emotionally erratic and back again, took the first of two championship points when Thiem pulled a forehand wide.With Thiem getting plenty of support throughout on Rod Laver Arena, Djokovic celebrated by shushing some of the crowd with his finger on his lip and then pointing to his box.His latest triumph moves him within three of Swiss Roger Federer's all-time leading tally of 20 men's singles titles, with Spain's Rafael Nadal now only two ahead on 19.The Serb's fightback ensured the wait continues for somebody to break the stranglehold of the old guard, with Djokovic, Nadal and Federer having won the past 13 Grand Slam titles.No other player has won a men's major title since Switzerland's Stan Wawrinka won the US Open title in September 2016.Djokovic will also return to the top of the world rankings, replacing Nadal, when they are published on Monday.