Nick Kyrgios believes no other player will be able to emulate the career Serena Williams has enjoyed, after the grand slam great confirmed her retirement.
Williams went down 7-5 6-7 (4-7) 6-1 to Ajla Tomljanovic – Kyrgios' former partner – on Friday in the third round of the US Open.
That brought an end to her playing career, though the 40-year-old suggested she would leave the door open for a potential return.
Kyrgios, who defeated J.J. Wolf to set up a contest with reigning men's champion Daniil Medvedev, is in awe of what Williams has achieved, as he ranked the 23-time grand slam champion alongside Rafael Nadal, Roger Federer and Novak Djokovic.
"She's had a career that I don’t think no one else will emulate," Kyrgios said.
"You've got a couple on the men’s side who are trying to catch her in grand slams, but Nadal, Serena, Federer, Novak, these are once in a generation athletes – I wouldn't even put myself in the same category, ever.
"It's a special moment for her, she played extremely well, she won two matches and nearly got on top [against Tomljanovic], it's a hell of a way to go out.
"I just appreciate what she's done and I'm sure everyone in this building does as well."
After beating Wu Yibing, world number one Medvedev said of Williams: "That was a crazy match, it was close to being three hours.
"It's definitely a pity that she lost. If it's the last match of her career, it was definitely an amazing match and she was really close to winning.
"Ajla played at a great level, it was not easy against the crowd and it was a high-level match."
Williams bows out with the most grand slam wins in the Open Era of any player, male or female.