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All the talk before the tournament had been about Novak Djokovic and whether the world number one would be able to compete to achieve the same feat, but it was Nadal who secured the historic victory at Rod Laver Arena, beating Medvedev 2-6 6-7 (5-7) 6-4 6-4 7-5 in a marathon five hours and 24 minutes.
It is only the second Australian Open title of Nadal's decorated career but puts him out ahead of Djokovic and Roger Federer (both 20) as the man to have won the most grand slams of all time.
He had looked down and out at times in the third set but showed typical determination to get better as the match went on, while Medvedev appeared to visibly tire as he saw his lead disappear into the night sky in Melbourne.
The first set began with Medvedev asking questions of Nadal, although initially the 35-year-old had answers with some classic forehand winners.
However, the unforced errors from the Spaniard began to pile up and he was broken to love in the fifth game. From there, Medvedev dominated the remainder of the opening set, breaking again and taking it 6-2.
It did not bode well for Nadal, who had won only three of his 10 prior major finals in which the opener had gone to his opponent.
The number six seed was struggling on his first serve, getting just 54 per cent in – his next lowest in a first set in this tournament had been 66 per cent in the second round win against Yannick Hanfmann.
Nadal showed some resilience, though, and hit a sensational winner at the end of a 40-shot rally in the fourth game of the second set, in which he ultimately broke Medvedev for the first time, only to be broken back to 4-3 as those serving struggles continued.
A back-and-forth affair saw four breaks of serve and the set ended with a tie-break, which Medvedev clinched with a backhand winner down the line to leave Nadal looking down the barrel of a defeat.
However, Nadal was not going to go down without a fight and showed some of his trademark grit in the third to stay with Medvedev, who was, if anything, playing even better than in the first two sets. Nadal had to save break points in the sixth game to eventually hold serve, before breaking in the ninth and serving out to somehow get back to within a set.
The drama did not stop in the fourth as two holds of serve were followed by three straight breaks to put Nadal 3-2 ahead. Both men were forced to save multiple break points thereafter, but Nadal successfully held serve to take it 6-4 and force a decider.
Medvedev looked to be wilting and was hanging on at the start of the fifth, before some superb Nadal winners earned a break in the fifth game.
The Russian made his opponent work hard for his victory and dramatically broke back to level when Nadal was serving for the championship, only for the veteran to break straight back before finally sealing the win and his place in the history books with a backhand volley that Medvedev could not return.
DATA SLAM: No Melbourne misery for Nadal
Nadal also becomes the second man in the Open Era – and only fourth in history – to win each grand slam at least twice, after Djokovic, Roy Emerson and Rod Laver.
Despite still boasting an impressive overall record in grand slam finals at 20-8 going into this match, Nadal was 1-4 in Australian Open finals. He looked sure to make that 1-5 after the first two sets but showed remarkable fortitude to turn things around.
This was Medvedev's second Australian Open final defeat having lost to Djokovic last year, and his second grand slam final defeat to Nadal after losing to him at the 2019 US Open.
Nadal – 69/68
Medvedev – 76/52
Nadal – 3/5
Medvedev – 23/5
BREAK POINTS WON
Nadal – 7/22
Medvedev – 6/22