To be blunt, hitting partners aren’t necessarily known as the headiest bunch. More often than not it’s a case of brawn over brains. Leave the Xs and Os to the coach. You’re here to be a backboard, not a sounding board.

Aleksandar Bajin toiled away on the ITF Futures circuit between 2005 and 2007, stalling at a career-high ranking of No. 1149, before receiving an offer to serve as a hitting partner for an American player of some repute: Serena Williams. Over the next eight years, as Williams amassed Slam title after Slam title, Bajin would gain some notoriety as one of the American’s closest confidantes. The New York Times went so far as to dub the Serbian-born German “Serena Williams’ Secret Weapon.” But there were some outside the inner-circle who viewed the sculpted Sascha as more beefcake bodyguard than skilled strategist.

Fast-forward to 2018, past Bajin’s split with Williams, past his stints with Victoria Azarenka and Caroline Wozniacki, and the onetime wannabe touring pro has become an insightful and well-respected coach, his new pupil none other than Naomi Osaka, who on Wednesday advanced to her maiden Grand Slam semifinal at the US Open, the first Japanese woman to reach the Final Four at a major in more than two decades.

“Since I was working with him — and I tend to be a bit negative on myself — I feel like I've gotten a little bit more optimistic,” said Osaka, whose path to the semis included 6-0, 6-0 and 6-1, 6-1 shellackings of Aliaksandra Sasnovich and Lesia Tsurenko, respectively. “He tries to make every day really fun and exciting. For someone like me, that sort of thinks sometimes things are boring, that's good for me.”

“I fight myself a lot, so he's sort of been, like, the peacemaker,” she added.

play video AI Match Highlight: Osaka vs. Tsurenko - QF

“There were a couple of matches in the past, it's not a secret, where she would get a little bit negative and down on herself just because things weren't going her way,” said Bajin. “But I think slowly she's finding a way to get herself out of it and not allowing so many negative emotions in her life. She's just doing it because she wants to do so well and she wants to be better. Some people feed off it. If you would have told John McEnroe, ‘You have to be positive,’ he'd be looking at you, like, ‘What are you talking about?’ To him it really helped. For Naomi, I believe she needs that positive mindset on court.”

There were plenty of positives this past spring in the Southern California desert, where Osaka, 20, scored the biggest title of her career, claiming the BNP Paribas Open with wins over the likes of Maria Sharapova, world No. 5 Karolina Pliskova and top-ranked Simona Halep. She doesn’t come off as the most physically imposing player on the court, but as we learned in Indian Wells, she can hit as big as anyone on the WTA Tour. In fact, her power game is somewhat reminiscent of Bajin’s former employer, though he insists their only common characteristic is their hair.

“Big hair,” he quipped.

play video Press Conference: Sascha Bajin

“I think they really are different people,” he said. “I believe that they kind of want to play the same. They are very powerful, big serves, big hitters, both of them. But even on court, Serena is very aggressive, and Naomi — I have to push her to get a fist-pump out of her. The mindset is different from one to the other. Off court, too, Naomi is a little bit more, reserved, just a little bit more shy.”

It’s not as if Bajin has been trying to mold a Serena clone.  

“She's been a big hitter before I started with her. She had this power. It's not that I added to her,” he said. “She knew how to play tennis. She maybe didn't know quite how to handle it or control it; didn't quite know when to pull the trigger, when not to; maybe didn't know that there were even other ways of putting pressure on the opponent by just taking pace off the ball. So I was trying to kind of maintain the raw power, and then, at the same time, also show that there are other ways of creating pressure. But they are completely different, Serena and Naomi.”

Bajin first met Osaka when he was working with Wozniacki. The American-born daughter of a Haitian father and Japanese mother joined them for a practice session at Roland Garros. Bajin confided that he initially found her a bit too standoffish for his taste.   

“I thought she was a little bit more of a diva because she didn't talk much,” he said. “She doesn't really look at someone's eyes, but that's just because she was always so shy. She would just keep her head down a little bit, which is cute now. Back then I didn't know for what reason. I feel bad for prejudging. But I'm very happy that it wasn't what I thought, that it was a positive surprise. Looking back, maybe it was meant to be, you know. Things happen for a reason. I'm glad I could help her.”

Osaka has endeared herself to fans as one of tennis’ quirkiest characters; a young-in-age/young-at-heart breath of fresh air who’s still growing accustomed to life in the spotlight. Cue up her trophy speech in Indian Wells if you need evidence of her lack of PR polish. Bajin says he’ll do all he can to preserve that sense of innocence, that shyness, something of a rarity in the sometimes dog-eat-dog world of professional tennis.     

“I believe the more open we are and the more honest we are and show vulnerability sometimes and who we truly are, the better this world is just going to be,” he said. “And all the fake emotions, I'm not a fan of it. I believe that she's a star for that.”

Wouldn’t it be something if his rising star and his former hitting partner, Serena, situated on opposite sides of the draw, met in the final of the 2018 US Open?

Said Bajin, “That would be beautiful.”