Duckworth had a grip on both sets against the young Italian on Friday but saw a set point in the first disappear and then couldn't repel Sinner's late surge in the second as he suffered a 7-6 (7-4) 6-4 loss that left him chuntering unhappily to himself at the end of the match.

It spelled the end of the Australian challenge in the ATP hard court event in the Bulgarian capital after John Millman had also earlier been beaten 6-4 6-2 in the last-eight by American Marcos Giron.

Duckworth's consolation after a third straight excellent week's work, which saw him reach a maiden ATP final in the Astana Open following a Challenger tournament triumph in Istanbul, is that he'll next week officially become Australia's No.2 men's player.

That accolade, though, hardly looked as if it would appease the 29-year-old Sydneysider, who had to rein in considerable frustration in his one hour 54 minute defeat by world No.14 Sinner, the defending champion and top seed in Sofia.

Duckworth, now up to a career-best 56 in the rankings after starting the year outside the top-100, had beaten the 20-year-old Italian in their last meeting in the ATP Canadian Masters in August in Toronto and largely dominated their opening set again in Sofia.

He raced into a 3-0 lead, then a 5-2 advantage, as he was rewarded for a hugely aggressive approach.

Nine times over three games in the opening set, Duckworth stood just two points away from taking the stanza and he even had a set point at 6-5 only for Sinner to crash down a big delivery.

Duckworth was unhappy with a line call overrule by the umpire in that penultimate game which seemed to distract him as he lost the first four points in the breaker.

Still, he battled back to break Sinner in the opening game of the second and continued to attack, even earning a break point that would have moved him into another 5-2 lead.

But the Italian then really demonstrated his quality, rescuing that game before earning the key break back, when Duckworth gifted his first double fault of the match, and stepping up a gear to annex the last two games.

"It feels very good," Sinner said on court.

"It was a very, very tough match. I lost the last one against him and he's been ina very positive situation, so it wasn't easy for me."

Earlier, Millman's hopes of earning his second ATP Tour title had been extinguished in a slugfest with Giron, who again proved too strong on the big points just as he had when knocking out Australia's No.1 Alex de Minaur in straight sets the previous day.

The 32-year-old Millman had been aiming to reach his first tour-level semi-final since the Astana Open this time last year when he went on to lift his maiden title.

But his defeat marks the end of his spell as Australia's No.2 as he'll slip down the rankings next week behind Duckworth, Alexei Popyrin and Jordan Thompson and he'll become the fifth-ranked domestic performer.

Australian Associated Press