Coco Gauff entered the 2025 US Open with a significant storyline shadowing her campaign: her serve. Despite being the top-ranked American and reigning Roland Garros champion, Gauff's serve had become a liability, plagued by double faults. This prompted a significant change, as she hired biomechanics expert Gavin McMillan, known for his work with Aryna Sabalenka, to overhaul her serving technique.

Gauff's first test with her new serve came in the first round against Ajla Tomljanovic. The match proved to be a grueling encounter, lasting nearly three hours. Gauff acknowledged the difficulty of implementing such changes during a major tournament. "Really tough," she said, describing the process as "mentally exhausting". Despite the challenges, she recognized the long-term benefits of the adjustments.

The match against Tomljanovic was a rollercoaster. Gauff started slowly but recovered to win the first set 6-4. She then led in the second set before Tomljanovic fought back to win the tiebreak. In the third set, Gauff again gained an early advantage but struggled to close out the match, even facing the prospect of defeat while serving at 5-4. Ultimately, Gauff prevailed 6-4, 6-7 (2), 7-5.

Gauff's serving performance was a mixed bag. She finished with 4 aces and 10 double faults. While the double faults were still present, the numbers were an improvement from her previous matches, such as the 23 she had against Danielle Collins in Montreal. She won 66% of her first serve points and 44% on her second serve. Gauff consciously reduced her serve speed, prioritizing consistency and motion. Her first serve speeds averaged 88 mph in the first set, 97 mph in the second, and 101 mph in the third.

Gauff is aware that improving her serve is crucial for achieving her goals. She has been "literally serving until, like, my shoulder was hurting" in practice. MacMillan's approach focuses on balance, spinal stability, and proprioception, aiming to optimize her technique. Gauff trusts MacMillan's methods, citing the science and data behind his theories.

The pressure of the US Open, combined with the ongoing serve reconstruction, creates a unique challenge for Gauff. She expressed that making such a change during a Grand Slam isn't ideal and would have preferred to implement it during a WTA 500 event.

Despite the difficulties, Gauff's win against Tomljanovic demonstrated her resilience and fighting spirit. She improved her record to 36-12 for the year and kept her hopes alive for a second Grand Slam title in 2025. She faces Donna Vekic in the second round. With her sights set on potentially reaching the world No. 1 ranking, Gauff's journey at the US Open is as much about chasing a major title as it is about refining her serve.