The last Aussie standing in the high-quality ATP 250 tournament outlasted Spanish luminary Roberto Bautista Agut, battling back from the brink of defeat to earn an exceptional 3-6 7-6 (7-5) 6-4 win in Wednesday's round-of-16.
It was the 29-year-old's best victory since he defeated the then world No.13 Diego Schwartzman in one of the shocks of the tournament at last year's Australian Open.
Bautista Agut has dropped to No.28 in the world but the 34-year-old remains a tough nut to crack and it appeared he was going to record a hard-fought but relatively straightforward win as he served for the match in the second set at 5-4.
But world No.94 O'Connell broke back, survived a couple of break points of his own in the next game and took the marathon set into a tiebreak, which he largely controlled.
In the decider, Bautista Agut looked to have weathered an early storm after going 2-0 down and earning a quick break-back, but O'Connell eventually earned the decisive breakthrough in the ninth game, before serving out for victory after a gruelling two hours 39 minutes.Â
It's set up the prospect of a quarter-final tie for the Aussie with former world No.1 Daniil Medvedev, who was playing Britain's Liam Broady later on Wednesday.
Earlier, O'Connell's compatriot Jason Kubler had been unable to reprise the best win of his career over Felix Auger-Aliassime.
Former world junior No.1 Kubler's dramatic rise last year had been highlighted by his first-ever victory over a top-10 player when he defeated the No.9 Auger-Aliassime at the Newport Open last July.
But in his first ATP main-tour tournament since lifting the Australian Open doubles crown with Rinky Hijikata in January, the chance of the Queenslander repeating his win over Auger-Aliassime in the last-16 disappeared after Kubler had raced into a one-set lead in the Qatari capital.
Kubler eventually didn't have the firepower to trouble the Canadian, who's still at No.9 in the world after an indifferent start to the season, going down 4-6 6-1 6-4 in just over two-and-a-quarter hours.
Things had begun promisingly when the 29-year-old Australian had taken advantage of Auger-Aliassime's returning struggles, winning 80 per cent of the points on his serve in the opening stanza.
But once Auger-Aliassime had found his range, there was never much doubt as the Canadian broke Kubler's delivery three times to set up a quarter-final clash with Spain's Alejandro Davidovich Fokina, who beat Korean Soonwoo Kwon 6-3 6-2.
The defeat was a disappointment for Kubler but his ranking still looks poised to rise again from his career-best No.74 after he beat Aslan Karatsev, who had to withdraw with injury when a set down, in the first round.
In the day's blockbuster three-hour tie, the indefatigable Briton Andy Murray once again defied his 35 years and metal hip to outlast the No.4 seed Alexander Zverev with a 7-6 (7-5) 2-6 7-5 win.
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