At the 2017 European Junior Swimming Championships in June and July, Grousset won bronze medals in the 50 metre freestyle, with a 22.59, and the 50 metre butterfly, with a 23.88, and placed seventh in the 100 metre freestyle with a 49.85 and fourth in the 4×100 metre medley relay, where he contributed to a final time of 3:41.67.[5] The following month, at the 2017 World Junior Swimming Championships, he won the silver medal in the 50 metre freestyle with a time of 22.25 seconds, placed seventh in the 100 metre freestyle with a 49.95 and sixth in the 4×100 metre medley relay with a 3:41.89.[6] He competed in the 4×100 metre mixed freestyle relay event at the 2018 European Aquatics Championships, winning the gold medal.[7]
The second day of the 2021 World Short Course Championships in December in Abu Dhabi, United Arab Emirates, Grousset placed sixth in the 4×50 metre mixed freestyle relay, swimming a 21.17 for the lead-off leg of the relay to contribute to a final time of 1:30.06.[13] On the morning of day four, he placed seventeenth in the preliminaries of the 50 metre butterfly with a time of 22.94 seconds, not advancing to the evening semifinals.[14] In the evening, he placed sixth in the final of the 50 metre freestyle, finishing in a time of 21.08 seconds.[15] Two days later, he started the evening finals session off with a fifth-place finish in the 100 metre freestyle in a time of 46.20 seconds.[16] He concluded the day, and the Championships, with a relay team disqualification in the final of the 4×100 metre medley relay, with the disqualification due to a negative reaction time for the start by the breaststroke swimmer.[17]
Grousset started competition on day one of the 2022 European Aquatics Championships, held in Rome, Italy, ranking second in the preliminaries of the 50 metre butterfly with a time of 23.20 seconds and advancing to the semifinals.[23] In the semifinals, he ranked second across both heats, qualifying for the final with a personal best time of 22.90 seconds.[24][25] Day two of competition, in the morning preliminaries of the 100 metre freestyle, he ranked fourth and qualified for the evening semifinals with a time of 48.31 seconds.[26] In the evening session, he won the silver medal in the 50 metre butterfly with a time of 22.97 seconds, 0.08 seconds behind gold medalist Thomas Ceccon of Italy.[27][28][29][30] Later in the session, he lowered his time to a 48.15 for the semifinals of the 100 metre freestyle and qualified for the final ranking fifth.[31] In the final of the 100 metre freestyle the following day, he placed fourth with a time of 47.78 seconds, finishing 0.15 seconds behind bronze medalist Alessandro Miressi of Italy.[30][32][33]
On the fourth day, Grousset led-off the 4×100 metre freestyle relay in the final with a 47.69 before the relay was disqualified for an early start by the second swimmer.[34][35][36] The next day, he swam the lead-off leg of the 4×100 metre mixed freestyle relay, contributing a 48.02 to the final mark of 3:22.80 to win the gold medal.[37][38] For the preliminaries of the 50 metre freestyle in the morning on day six, he tied Diogo Ribeiro of Portugal for fourteenth rank with a time of 22.23 seconds and qualified for the semifinals.[39][40][41] In the evening semifinals, he brought his time down to a 21.75, ranking fifth overall and qualifying for the final.[42][43] The seventh day, he placed fifth in the final of the 50 metre freestyle with a time of 21.87 seconds.[44] Later in the session, he won a silver medal as part of the 4×100 metre medley relay, anchoring the relay to a finish in 3:32.50 with a split time of 47.43 seconds.[45][46]
In the 100 metre freestyle at the 2022 FINA Swimming World Cup stop held in Berlin, Germany, Grousset won the silver medal with a time of 46.38 seconds, finishing 0.50 seconds behind gold medalist Kyle Chalmers.[47] For his other events, he placed sixth in the 50 metre freestyle with a time of 21.31 seconds and ninth in the 50 metre butterfly with a time of 22.98 seconds.[48]
Achieving a personal best time of 45.77 seconds in the preliminaries of the 100 metre freestyle on day two of the 2022 World Short Course Championships, held in December in Melbourne, Australia, Grousset qualified for the semifinals ranking second overall.[49] In the evening semifinals, he lowered his personal best time to a 45.58 and qualified for the final ranking second.[50][51] The morning of day three, he ranked first across all preliminary heats in the 100 metre individual medley with a time of 51.94 seconds and advanced to the semifinals.[52] Finishing in a personal best time of 45.41 seconds in the evening final of the 100 metre freestyle, he won the silver medal.[53][54][55][56] He was subsequently disqualified in the semifinals of the 100 metre individual medley in the same session.[57]
The morning of day four, Grousset split a 20.76 for the second leg of the 4×50 metre mixed freestyle relay to help advance the relay with a time of 1:29.69 to the final ranking first.[58] Then he tied in rank for tenth in the preliminaries of the 50 metre freestyle with a time of 21.13 seconds and advanced to the semifinals.[59] In the evening session, he led-off the finals relay with a 20.92 to contribute to a new world record time of 1:27.33 and winning the gold medal.[60][61][62] Approximately 65 minutes later, he ranked eighth in the semifinals of the 50 metre freestyle with a personal best time of 20.97 seconds and qualified for the final.[63] The following morning, he split a 22.18 for the butterfly leg of the 4×50 metre medley relay, helping qualify for the final ranking second with a 1:32.53.[64] On the finals relay, he lowered his time to a 21.90, helping achieve a fifth-place finish in a time of 1:31.41.[65][66] Later in the session, he placed fifth in the final of the 50 metre freestyle, finishing in a personal best time of 20.90 seconds.[66][67] On the sixth, and final, day, he achieved a personal best time of 1:41.79 in the preliminaries of the 200 metre freestyle to qualify for the evening final ranking third.[68] Improving his personal best mark to a 1:41.56 in the final, he placed sixth.[69][70]
At the 2023 Camille Muffat Meeting in March in Nice, Grousset won the silver medal in the 100 metre freestyle with a 48.97, finishing 0.02 seconds behind Szebasztián Szabó of Hungary.[71][72] Three days later, he won the gold medal in the 100 metre freestyle at the 2023 Giant Open with a time of 48.50 seconds.[73][74] The day before, 18 March, he won the 100 metre butterfly with a personal best time of 51.72 seconds.[72][74] On 11 June, day one of the 2023 French Elite Swimming Championships in Rennes, he won the gold medal in the 50 metre butterfly with a time of 23.06 seconds.[75][76][77] On the third evening, he won the gold medal in the 100 metre freestyle with a 2023 World Aquatics Championships qualifying time of 47.62 seconds, which also ranked him as the third-fastest performer in the event for the 2023 year, 0.40 seconds behind first-ranked Pan Zhanle of China.[78][79] Two evening sessions later, he won the silver medal in the 50 metre freestyle with a World Championships qualifying time of 21.78 seconds, finishing 0.16 seconds after gold medalist Florent Manaudou.[80]