Caroline Wozniacki flickers on and off on way to semi-finals

Caroline Wozniacki's career is as hard to read as the unconventional game of Su-Wei Hsieh, 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 here for the second time in five years.

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

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 $25 million. She always seems on the verge of ruling her sport, yet, at 27, she is yet to win a Grand Slam.

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

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In the day-time quarter-final, the Belgian Elise Mertens, ranked 37 in the world, had too much for world No 4 Elina Svitolina, winning 6-4, 6-4 in an hour-and-a-quarter. It looked like another authentic upset but Svitolina revealed she'd 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 later. “Not expected, especially today. Really tough match. But I was in the zone today. 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.”

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. – Guardian service