• No 2 seed will face unseeded player after beating Navarro • Dane is two matches away from her first grand slam title

Caroline Wozniacki’s career is as hard to read as the unconventional game of Hsieh Su-wei, and the Dane’s seeming return to consistency lurched sideways then back on track as she stumbled into the semi-finals of the Australian Open for the first time since 2011.

It was easy to forget, watching the first set of her quarter-final against Spain’s world No 39, Carla Suárez Navarro, that, in the second round the previous Wednesday, Wozniacki, the No 2 seed, had to win six straight games and save two match points after going 1-5 down in the third set against the world No 119 from Croatia, Jana Fett.

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That is an insane swing of fortune and form for a former world No 1 who last year beat three No 1 players and had the biggest win of her career in the WTA Finals in Singapore, lifting her career prize money past $25m when she beat Venus Williams. She always seems on the verge of ruling her sport, yet, at 27, she is yet to win a slam.

So, in part, a scoreline of 6-0, 6-7 (3), 6-2 is wholly in keeping with her career, flickering on and off like lights on a Christmas tree.

Wozniacki’s opponent in the last four will be Elise Mertens, the Belgian ranked 37 in the world who had too much for the world No 4, Elina Svitolina, in the daytime match, winning 6-4, 6-4 in an hour and a quarter. It looked like another authentic upset but Svitolina revealed she had brought a hip injury into the tournament – the injury du jour in the game. It struck Jo Konta down in Brisbane, which is where Svitolina first felt discomfort.

“It was getting worse and then was up and down,” she said. “I always had heavy tape under the shorts. Sometimes it was fine, but today she played a good level, so I had to push myself. She didn’t give me opportunities. All credit to her, because she played really good tennis.”

Mertens was taking the win whatever way it arrived. “It’s amazing,” she said. “Not expected, especially today. Really tough match. But I was in the zone. I played really well.” She reckoned it was the biggest win of her career, adding: “She’s a great mover. She can do great things, defence but also offensive.”

Wozniacki was full of praise, saying of Mertens: “She’s had an amazing start to the year, I think she’s undefeated. It’s going to be tough one but I’m excited for it.”

While it might be regarded as a surprise if she could pull off back-to-back wins against Svitolina and Wozniacki, this tournament has been riddled with shocks from day one. On this side of the draw alone, six seeds fell in the first round, four in the second, two in the third, two in the fourth and Svitolina in the quarters.

In the other half, seeds also tumbled throughout the first week: 11 in all, making a total of 26 from the starting line-up of 32. Angelique Kerber was the only slam champ standing after the fourth round.

That is serious carnage. So, predictions are tough to make. Can Mertens beat Wozniacki? Probably not but not many people thought Venus Williams, Sloane Stephens and CoCo Vandeweghe would be off the slate after day one. It has been far from dull.