The hype around Serena Williams' potential swansong tournament is "the biggest thing I've ever seen in women's tennis", says Torben Beltz, the coach of her next opponent Anett Kontaveit.

Williams has announced she is soon to retire, and the US Open is widely expected to be her final tournament.

The 23-time grand slam champion came through in straight sets in her first-round match against Danka Kovinic to set up a tough meeting with world number two Kontaveit on Wednesday.

Beltz wanted to watch Williams' opening match but could not do so after Arthur Ashe Stadium sold out, with reports of tickets on the secondary market on sale for around $1,000.

And Beltz says he has not seen anything like it, telling the New York Times: "I couldn't get a ticket. 

"This is the biggest thing I've ever seen in women's tennis. I think it's the greatest thing for the sport, and we all have to thank Serena for all she did. Especially right now with the end coming."

While Beltz's charge Kontaveit is favourite for the second-round match, the Estonian's coach knows it will be a tough match against the six-time US Open champion.

"I think her ball speed, serve and return is really up to her prime time," Beltz added.

"I saw her other matches, and it looks like she's improved over the last couple of weeks. She looks in better shape and looks good now.

"For Anett, I think the key is to just go out and try to play her best tennis but also enjoy the moment. It's going to be a big challenge, a great challenge, but I think she wants that challenge and wants to embrace it."

Emma Raducanu hailed Torben Beltz as "one of the nicest people I've met", despite deciding to split from the German coach.

The US Open champion parted company with Beltz on Tuesday after just five months together.

The duo began working together in November but have now split as the world number 11 plots the way forward ahead of the French Open and Wimbledon.

"Torben is a great guy. I really enjoyed my time with him on and off the court," Raducanu said while preparing for the Madrid Open on Wednesday. 

"He is one of the nicest people I've met, so obviously it was a tough one to split with someone like that.

"But I feel like right now I'm very comfortable with my current training. I'm feeling very confident in what I'm doing and how I'm working.

"I feel like over the last few weeks it's definitely become more apparent and especially as I've spent more time on the tour playing more matches against these top opponents, that I kind of understand what I feel like I need more of.

"I think Torben has been great for me because when I wanted someone with tour experience, I think for my first six months on the tour, it was very valuable."

The 19-year-old has recently enjoyed her best week of the season, winning back-to-back matches at the Stuttgart Open, before putting in a respectable performance in defeat to world number one Iga Swiatek.

Beltz is the third coach to move on from working with Raducanu in the last 12 months. She swapped Nigel Sears for Andrew Richardson, who worked with her at the US Open, before deciding to bring in Beltz, who previously worked with Angelique Kerber and Donna Vekic.

Raducanu is scheduled to face Czech player Tereza Martincova on Friday in Madrid and will be assisted by Iain Bates at the tournament, the long-serving head of women's tennis at the Lawn Tennis Association.

Emma Raducanu has parted company with another coach as the US Open champion seeks the right combination to keep her at the top of the game.

The world number 11, who was a shock winner at Flushing Meadows last September, announced on Tuesday she and Torben Beltz would no longer be working together.

German coach Beltz came on board in November but departs as Raducanu plots the way forward ahead of the French Open and Wimbledon.  The 19-year-old Raducanu says she needs "a new training model".

Beltz becomes the third coach to move on from working with Raducanu in the last 12 months. She swapped Nigel Sears for Andrew Richardson, who was in her corner at the US Open, before electing to bring on board the experienced Beltz, who previously worked with Angelique Kerber and Donna Vekic.

Raducanu said of her decision: "I want to thank Torben for his dedication. He has a huge heart and I have enjoyed our strong chemistry during the time together."

She is preparing to play at the Madrid Open and will be assisted there by Iain Bates, the long-serving head of women's tennis at the Lawn Tennis Association (LTA).

Raducanu has yet to indicate what direction she will go in regarding her next full-time hire, although working with the teenager is likely to remain an attractive prospect for leading coaches.

She said: "The best direction for my development is to transition to a new training model with the LTA supporting in the interim."

Although Raducanu has continued to climb the WTA rankings, she has managed just five wins from 12 matches on tour this year, losing a close contest to world number one Iga Swiatek in the Stuttgart quarter-finals last week.

Emma Raducanu has appointed Torben Beltz as her new coach almost seven weeks after parting company with Andrew Richardson.

The 18-year-old rose to international fame in September when she pulled off the incredible feat of clinching the US Open before ever even winning a single match on the WTA Tour.

Raducanu beat fellow teenager Leylah Fernandez to become the first qualifier in tennis history to win a grand slam and youngest winner of a women's major since 17-year-old Maria Sharapova took the Wimbledon title in 2004.

But she announced soon after that she and Richardson were no longer working together, with Raducanu continuing in the meantime without a coach as she scoured for someone with more WTA Tour experience.

Since the US Open, she has played just five matches and lost three of them, the latest coming on Tuesday as she suffered a 6-1 6-7 (0-7) 7-5 defeat to Wang Xinyu at the Linz Open, where Raducanu was the top seed.

But as she looks ahead to starting pre-season at the end of November, Raducanu will begin working with Beltz in the hope of building on her incredible breakthrough year.

The German was Angelique Kerber's coach during her US Open and Australian Open successes in 2016, and Raducanu is excited to get started.

Speaking after her defeat in Linz, Raducanu said: "You guys cracked it! It's obviously a great privilege to be working with such an experienced coach.

"I'm definitely very excited to work with him throughout the pre-season and into next year. I'm feeling excited about all the work that is to come.

"I think he brings a lot of experience. Obviously he's worked with Kerber, who's such a great player and has done extremely well, won three slams. I think that experience definitely helps with someone as inexperienced as me.

"He can help guide me through, which I feel really confident about, and also he's a really positive, cheerful guy who brings great energy to the team, so I think that's also important when you're travelling on the road for quite a long time on the tour."