Carving out what he was convinced was the biggest win of his largely unheralded career against the home idol Alexander Zverev on a chilly Munich lunchtime, Australia's archetypal battler O'Connell looked like an icy hitman in the BMW Open on Thursday.
The Sydneysider had the wrapped-up crowd at the outdoor event in Bavaria chuckling away as he told them afterwards it was the coldest he'd ever felt on a tennis court - and yet his form continues to bring a warm glow to Aussie tennis, desperately seeking players to make a run at next month's French Open.
At the moment, there looks to be two men capable of doing some damage at Roland Garros as both O'Connell and Alex de Minaur reached quarter-finals of European clay-court events on Thursday.
O'Connell had to do it the hard way, beating a German idol, Olympic champion and two-time ATP Finals winner Zverev in front of his adoring home fans.
Alex de Minaur, though, managed to make the last-eight at the Barcelona Open rather more easily, with his feet up in Catalonia after being given a walkover against Bulgarian rival Grigor Dimitrov, who withdrew through illness before their last-16 encounter.
It has set up an important Friday for both Sydneysiders, with O'Connell in with a golden opportunity to reach his first semi-final of the season when he's paired with Italian qualifier Flavio Cobolli.
In Barcelona, though, De Minaur will once again have his work cut out against Stefanos Tsitsipas, a man he's played eight times and lost to on every occasion.
The Greek star looked ominously impressive in beating Canadian Denis Shapovalov 6-3 6-2 on Thursday, while Carlos Alcaraz also looked in good shape, overcoming windy conditions to beat fellow Spaniard Roberto Bautista Agut 6-3 7-5.
Back in Munich, O'Connell was able to tick off his biggest scalp yet as he powered to a 7-6 (7-4) 6-4 win in four minutes under two hours, quite ruining Zverev's 26th birthday.
Zverev may not currently be ranked as high as Diego Schwartzman, who was world No.13 when O'Connell put paid to the Argentine in last year's Australian Open, but he's still one of the biggest names in the sport and the Aussie knew the significance.
"Yeah, for sure," beamed O'Connell, after being asked on court whether his victory over the world No.16 was the biggest in his six years as a pro.
"It's definitely hard playing Zverev here in front of his home crowd.
"I'm definitely not prepared to play in this weather - this is the coldest I've ever played in, it was tough! Took me a while to get warmed up."
Zverev never did, though. "In the last few years I can't really handle the pressure of playing in Germany. I'm incredibly nervous. In the games I'm nowhere close the level I show in training," he said afterwards.
"I put the pressure on myself. It doesn't come from outside,"
Zverev was covered up, wearing tracksuit bottoms beneath his shorts, but O'Connell, ranked No.82 in the world, looked much happier to be out there as he always seemed in control.
There was a brief spell in the second set when he broke back from 4-2 down to make it 4-4 that the German looked like the real Zverev.
But he then stuttered again, serving a fifth double fault before being beaten by a dazzling cross-court winner from O'Connell and a ripped forehand winner down the line which set up the Aussie's clinical finish.
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