Who achieved a stunning upset by beating Coco Gauff at the US Open?

The correct answer is Naomi Osaka, who achieved a significant upset by defeating Coco Gauff at the US Open. This match drew considerable attention not only for the competitiveness and skill displayed but also due to the narrative surrounding the two players, as Gauff was seen as a rising star in women's tennis while Osaka has been a prominent figure and former Grand Slam champion. The victory highlighted Osaka's resilience and ability to compete at a high level, further solidifying her status as one of the leading players in the sport.

The other players mentioned, such as Serena Williams, Venus Williams, and Ashleigh Barty, have notable records and accomplishments but were not the ones who achieved this specific upset against Gauff in the context of the question. Each of these players has had their own moments of triumph in tennis, but they were not part of this particular upset at the US Open.