Caroline Wozniacki had Emma Raducanu's back when hitting back at John Isner's claim about her remarkable US Open title triumph. It was in 2021 when the Briton became the first-ever player to win a Grand Slam as a qualifier during her run at Flushing Meadows.

The 23-year-old, who was a teenager when she lifted the title, won 10 matches in straight sets en route to her tournament win. Ranking among the greatest stories in tennis history, the sport witnessed another stunning upset just last month when Valentin Vacherot became the lowest-ranked Masters champion in history when he won the Shanghai Masters.

The Monegasque star beat the likes of Alexander Bublik, Holger Rune and Novak Djokovic on the way to the final where he defeated his cousin Arthur Rinderknech to lift the title. While both Raducanu's and Vacherot's successes were hugely awe-inspiring, Isner claimed the latter's triumph ranks more impressively than the Brit's Grand Slam victory.

The former American tennis star, who was ranked world No. 8 during his career, made the claim on the Nothing Major Show following Vacherot's victory. He said: "I think it is more impressive, yes. (For Raducanu) it's a bigger story because it's the US Open, but I think this is more improbable than what she did."

In his argument backing Vacherot, Isner pointed out that he had just one main-level win to his name at 26 years of age before his run in Shanghai. However, he received pushback from Wozniacki during her appearance on the podcast who said claiming a Grand Slam title reigns supreme over any other achievement.

"No matter how you slice and dice it. That's better than winning a 1000... When you just talk about the result, the end result, i think winning the US Open is better than winning a 1000 Shanghai," the 2018 Australian Open champion said. The 35-year-old called time on his tennis career in 2022 with her last match being at the Australian Open in a third-round defeat to Ons Jabeur.

The Dane ended up walking back her decision and returned to the sport in 2023 where she notably competed in the US Open as well as the Indian Wells Opena and the Paris Olympics last year. Wozniacki, 35, welcomed her third child back in July and while she has yet to confirm her retirement, it remains to be seen whether she picks up a tennis racket again.