Australian No.1 de Minaur dragged himself back from match point down to fight deep into the deciding set in the last-16 encounter, only for Nishikori to raise his game to seal a 6-3 2-6 7-5 victory.

It was a disappointment for de Minaur, who had been hunting his second tournament win of the year, as the 31-year-old Nishikori defied palpable weariness to produce his best at the end of two-and-a-quarter hours of the exhilarating and draining encounter.

The former world No.4 and US Open finalist eventually prevailed on his third match point to reach his first quarter-final at an ATP Tour event since the summer of 2019.

De Minaur had gone into the match in buoyant mood after beating John Millman in the battle of Australia's top two men players in the previous round but he started slowly, with erratic groundstrokes producing 14 unforced errors in the opening set.

The world No.23 de Minaur, ranked 22 places ahead of Nishikori, hit back fiercely in the second, breaking to love in the third game and finding deeper strokes that enabled him to dictate terms.

Nishikori, who's had elbow and shoulder injuries as well as dealing with COVID-19 during a difficult couple of years, began to appear fatigued but managed to edge a seven-minute first game in the final set which turned the momentum.

De Minaur saved a match point with a flashing backhand on Nishikori's serve at 5-3 down before squaring things up at 5-5.

It only inspired Nishikori to find a new level and he broke serve to advance to the next round.

"I am happy to be in the quarter-finals at this tough tournament," Nishikori said.

"I just need to keep going and be ready for the next match."

His next opponent will be Borna Coric or Dusan Lajovic, who caused the shock of the day by knocking out Australian Open finalist Daniil Medvedev 7-6 (7-4) 6-4.

It was not the only shock of the day as Kazakhstan's Alexander Bublik, who'd been beaten by Australian Alexei Popyrin in Sunday's Singapore Open final, accounted for German third seed Alexander Zverev 7-5 6-3 and Alejandro Davidovich Fokina eliminated fellow Spaniard Roberto Bautista Agut, the fifth seed, 6-2 7-6 (7-3).

Andrey Rublev avoided the cull with a 7-5 6-2 win over a frustrated former world No.1 Andy Murray sending the Russian fourth seed into the last eight.

Murray, looking for only his second win over a top-10 player since returning from the career-saving hip surgery he underwent two years ago, exclaimed late in the first set, "I'm totally unprepared for this."

Meanwhile, sixth seed David Goffin and American Tommy Paul progressed to the round of 16.

Australian Associated Press