A weary John Millman was able to raise a smile after battling through a three-hour, three-set battle to keep alive his hopes of retaining his prized Astana Open title in Kazakhstan.

Last year's triumph meant the world to the Australian No.2 as he clearly demonstrated once again in Nur-Sultan on Wednesday in a tenacious victory over Spanish grinder Jaume Munar.

Their last-16 encounter lay in the balance until Millman earned a crucial break to love in the penultimate game and then finally served out for victory, but still only nailed his quarter-final place 6-4 6-7 (5-7) 7-5 on his fourth match point after yet more late resistance from Munar.

The three-hour eight minute marathon was the last thing even the super-fit Millman must have fancied after another lengthy three-set contest in the first round, admitting afterwards: "These long matches aren't good in my old age, I tell you!

"It was such a physical battle out there. I thought the level was extremely good. But Mr. Munar, he's a tough competitor.

"His body probably let him down a little bit towards the end. I've got to tell you, my body wasn't feeling all that great, either. It was such a physical match and just so happy to extend my stay here."

At 32 and a battle-hardened veteran of the tour, the Queenslander admits it's been a "challenging" year for him as he's had to struggle with a back injury that scuppered his French Open bid and a badly bruised foot that disrupted him more recently at the Olympics and in the build-up to the US Open.

But while also in between showing flashes of his best form, Millman has come to Kazakhstan in the best of heart, returning to the scene of his only ATP triumph and now taking his record at the Nur-Sultan hard court tournament to 6-0.

In the next round, the fifth seed Millman could face Australian No.3 James Duckworth, who maintained his own career-best form which has propelled him to No.65 in the world by defeating Kazakh home favourite Mikhail Kukushkin 7-6 (7-3) 6-1.

Sydney's 'Ducks' has to get past Serbian world No 37 Filip Krajinovic in Thursday's last-16 in their first meeting if he's to set up an all-Australian quarter-final with Millman, who won their only previous meeting.