NEW YORK, NY, USA - Twenty years after winning her first Grand Slam singles title at the 1999 US Open, Serena Williams is still playing and training with the same intensity that she had as a teenager, according to her former hitting-partner and confidante Sascha Bajin. 

In an exclusive interview with wtatennis.com, Bajin spoke of his admiration for how Williams has maintained such a high level of tennis over the last 20 years, despite "the ups and the downs" and "the setbacks" she has experienced since beating Martina Hingis in that all-teenage final in New York.

Read more: 20 years on at Flushing Meadows: The story of Serena Williams' first Slam

In Bajin's analysis, Williams has made improvements to her game during that time, including enhancing her serve and her dropshot, but he has been most impressed by her mind and how she continues, ahead of her 38th birthday in September, to bring such focus, desire and discipline to the court. 

Such is Williams' astonishing longevity, she is attempting at Flushing Meadows to equal Margaret Court's all-time record of 24 Grand Slam singles titles.

"If you look at Serena back then in 1999, and you look at Serena now, there's not so much of a difference. It's not so much that her game has changed and developed into something else, and that she has added more variety or something. What I'm most impressed with is that she has maintained the level over the last 20 years," said Bajin, who worked with Williams from 2008 until 2015.

"She has somehow been able to maintain that level day in and day out, and really go all out every single day over a period of 20 years."

That intensity could propel Williams - who has lost her last three Grand Slam finals, including to Simona Halep at Wimbledon this year - to the US Open title.

"Every time Serena steps on the court, she sees herself walking off as the winner, and there's no doubt that, through the ups and downs, and the setbacks she's has had, she has maintained that same level of focus and determination and urgency and discipline - those are the things that I would highlight," said Bajin. 

As Bajin noted in his book, Strengthen Your Mind, which was published in Japanese this year, Williams is "a machine".

"Boredom is a problem for almost everyone at work. Even elite tennis players occasionally feel bored at work. That's inevitable when you're doing the same thing again and again, whether on the practice court or in the office. Your mind tires of the repetition, and you can feel your energy and enthusiasm draining away. About the only exception is Serena, who is capable of doing the same thing day in and day out without getting bored; that girl's a machine," observed Bajin, who coached Naomi Osaka to victory over Williams in last year's US Open final, and who is now working with Frenchwoman Kristina Mladenovic. 

If there has been one change in Williams' mindset over the last 20 years, Bajin said, it's that the American now sees she was destined for greatness.

"In 1999, Serena believed that she was destined for greatness, and now she knows it," he observed. 

For the most part, Williams will play the same game in New York this summer that she did as a 17-year-old in 1999.

"Back in 1999, she was already so consistent and so good. She had the angles, she had the shots. If I could point to one thing, it would be her serve. But nothing really physical. She doesn't really come to the net. She wasn't really comfortable coming to the net in 1999, and she isn't comfortable now. Maybe her dropshot is a little better now, but it's not really one of her shots," Bajin said.

"Serena's tennis hasn't changed that much in 20 years. Her game is her game."

Read more: 'Too exciting to compute' - How Serena won her first Slam title in 1999