South Korean Chung, who reached the Australian Open semi-finals previous year, fell at the first hurdle for the second tournament in succession, going down 7-5 6-3 to a player ranked 285 places below him.
· Karolina Pliskova won her second Brisbane title, coming back from a set and 5-3 down in the second set to beat Lesia Tsurenko, 4-6, 7-5, 6-2.
Now Medvedev hit back, saving multiple break points in the early stages of the second set and breaking in the eighth game, 5-3.
Medvedev, who saw off Jo-Wilfried Tsonga in the semi-finals, saved eight break points in the second set before capitalising on an unusually errant service game from Nishikori and levelling the match.
The victory followed a semi-final performance Nishikori described as "almost perfect" and came as a relief after he lost the decider in Brisbane two years ago.
"I expected something like this before the match - I'd seen him play a couple of times and I knew it would be tough", Tsonga said.
"A lot of my time is spent in the hotel lying in bed trying to conserve as much energy as I can, because whenever I step out on court there's so much I leave out there", he said.
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He said the story of the black pianist who strikes up a friendship with his Italian-American driver gave him hope. While collecting his Breakthrough Performance award at the ceremony, he said: "Thank you, Lucy Boynton ".
"I don't really agree it was up and down because I didn't lose one serve in the match".
"I always start with this tournament and Melbourne, [but I] couldn't make it previous year".
"I was a bit more aggressive than him and it made the difference", he said.
Reigning champion Daniil Medvedev may opt to skip his Sydney International defence following a strong campaign in Brisbane.
Medvedev withstood the huge serving of Canadian Milos Raonic to win a brutal match 6-7 (2/7), 6-3, 6-4.
"I think I returned well - that was the biggest key, I think".
"The main goal is to do well in the Australian Open, but I'm really happy that I've been playing really well this week", he said.