James Duckworth has had to put a close friendship aside as he defeated his great mate John Millman in an all-Australian battle to qualify for the Astana Open semi-finals in Kazakhstan and continue his major career breakthrough.

Sydney's unseeded Duckworth knocked out the defending champion and fifth seed Millman 6-4 6-4 to reach only his second semi at an ATP Tour event in Nur-Sultan on Friday.

It came just a few days after the world No.65 also won his first Challenger event for 19 months in Istanbul.

Seemingly in the form his life, the 29-year-old Duckworth won his eighth straight match to guarantee that his career-high ranking will push him into the world's top-60 for the first time next week.

Duckworth will next face Bulgaria's Ilya Ivashka - a 4-6 6-4 6-1 winner over Finn Emil Ruusuvuori in their last-eight match - in Saturday's semi-final as he seeks his first ATP title.

A maiden triumph on Sunday would see him overtake Millman as Australia's No.2.

"It was really tricky," Duckworth told the ATP website, as he reflected on his win over his 32-year-old Queensland pal Millman.

"John is a good player and we are really good mates.

"We practise at the same site. It is always difficult playing someone so close to you, but you have to put that aside and focus pretty hard and I was able to play a pretty good match."

Duckworth continued the superb serving that has carried him through the week, delivering nine aces and winning 78% of his first-serve points in the one-and-a-half hour triumph.

The pair had met just once before, six years ago in a qualifier for the 2015 China Open in Beijing, when Millman won in straight sets, but Duckworth quickly took a grasp on proceedings on the Nur-Sultan hard court after breaking the Queenslander's serve in the very first game.

Duckworth's rise has come while Australian No.1 Alex de Minaur's fortunes have slumped further as he suffered another early exit at the Moselle Open in Metz on Thursday

De Minaur, who's had a torrid time since a brilliant breakthrough grass court season saw him triumph in Eastbourne and move into the world's top 15, endured another first-match defeat in the French hardcourt tournament, losing 7-5 7-6 (7-5) to American Marcos Giron.