Aryna Sabalenka carries a heavy heart in defeat to Dayana Yastremska

Aryna Sabalenka’s heart has never been so heavy.

Aryna Sabalenka plays a return to Dayana Yastremska during their semi-final at the Adelaide International. Picture: AP

Aryna Sabalenka is six years old. Her father, Sergey, is driving around the back streets of Minsk, looking for something to do. They see an empty tennis court, go for a hit. Boom. Boom. The explosion of ball on racquet strings.

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For the next 15 years, father and daughter find bigger and better roads to explore, better and more prestigious courts to visit, taking on the world with her boom-boom forehand, her boom-boom backhand, her boom-boom serve and her boom-boom temperament. She wins last year’s WTA Elite Trophy, the off-Broadway yet prestigious event that has signalled, a year earlier, Ash Barty’s rise to Roland Garros glory and the world No 1 ranking. And then Sergey dies.

Now the 21-year-old is in the semi-finals of the Adelaide International. When Dayana Yastremska is asked what she expects from Sabalenka at Memorial Drive, she replies, “Boom, boom.” Because Sabalenka has always competed with her heart on her sleeve, even if the heart has never been this heavy. Because she’s always hit the ball so hard it may like to yelp in pain. She’s a heck of a young player with a heck of a strong personality: the tattoo of a tiger on her arm represents who and what she wants to be, as do the black stripes across her racquet.

Early round matches on lonely old back courts in Adelaide have nearly reduced her to tears. It’s been unclear why she’s been so emotional — until she reveals that Sergey has passed away in November. Few people know when she takes the court against Yastremska. “It was something unexpected,” she says. “He was young. He was 43. I didn’t want to say that but I think people should know. During the pre-season, it was tough to practise, mentally. I couldn’t leave, because I knew it was a tough situation with my family. I couldn’t leave them but somehow I needed to go. I’m trying to fight because my dad wanted me to be No 1. I’m doing it for him. That’s what is helping me to be strong right now.”

Sabalenka is virtually on her own in Adelaide. She’s split from her former coach, Dmitry Tursunov. Her family is still in Minsk. She’s here with her hitting partner and a fitness/psychology mentor. She’s had too much boom-boom for Wimbledon champion Simona Halep in the quarter-finals but the complication in the semi-final will be that Yastremska, a freewheeling 19-year-old Ukraine coached by Sascha Bajin, who’s previously been in the corners of Serena Williams and Naomi Osaka, has plenty of boom-boom of her own.

They slug it out. Big babe tennis. Sabalenka gets a polite ovation from the crowd. It would be a rousing reception if they knew the background. Yastremska’s boom-boom proves too much for Sabalenka’s boom-boom, 6-4 7-4 (7-4). She reaches her first Premier-tier final to suggest if Ash Barty is to remain the world No 1, or if Sabalenka is to get there for dad, they’re going to have Yastremska to deal with in the years to come. Already the world No 24, she’s on the up and up and up.

An off-season with Bajin in Spain has paid off. “I’m just really proud of myself that I could stay 100 per cent focused each practice, because it wasn’t easy,” she says. “Especially when you have, like, a week free, when you’re really tired physically, mentally, but you still have to keep going and work. Because after the pre-season, when you play tournaments, it’s the same thing. When you get really tired, you are going to have to give it a little bit more until the end.”

Bajin was Williams’ hitting partner before he became Osaka’s full-time coach. They won the US Open and Australian Open together but then she ditched him. Their personalities did not quite click. Asked about Bajin, Yastremska says: “Sascha’s philosophy? He’s different. He’s really different. Sometimes he can be really fun on court. If he sees that I’m tired and emotionally down, sometimes he can be really serious.

“The coach and player can find the right way with each other. Sometimes you just have to make fun. Sometimes you really have to push. And sometimes you have to get angry. I think he’s doing a good job in finding this balance. I have to try to stay realistic and play match by match, but I have big goal for the Australian Open. I’m going to keep it to myself.”

Yastremska has studied in detail her Australian Open draw, joining the dots, predicting the match-ups, coming up with a possible fourth-round clash against queen boom-boom, Serena Williams.

“I like to play in Australia,” she says. “I like it when it’s hot, when it’s humid. This year it will be different because of what is going on now in Australia, but I’m so happy to be here playing. I think the draw is very good — I’m going to try to get to Serena. I would really like to play her but like I said, match-by-match.”

Sabalenka moves on, or tries to. She keeps her heartache to herself. More power to her in Melbourne. “Not everything perfect right now,” she said.