2021: ATP and top 150 debuts, maiden Challenger title In January, Grenier made his ATP Tour main draw debut at the
2021 Antalya Open, where he defeated Slovak qualifier
Alex Molčan in the first round in straight sets before losing to fifth seed German
Jan-Lennard Struff by walkover in the second round. In November, Grenier won his first career
ATP Challenger Tour singles title in
Roanne, France.
2022: Major & top 100 debuts He entered the main draw as a
lucky loser at the
2022 Barcelona Open Banc Sabadell. He made his Grand Slam singles main draw debut also as a lucky loser at the
2022 Wimbledon Championships where he earned his first career Grand Slam singles main draw match win and just his second career ATP Tour singles main draw match win by defeating fellow first-time qualifier
Marc-Andrea Hüsler in five sets in a close-to-4-hours first round match. He lost to the unseeded
Cristian Garín in straight sets in the second round. Later in the month of July, he won his second career ATP Challenger Tour singles title at the
2022 Open Castilla y León dropping one set the whole week. In August, he made his
US Open singles main draw debut, entering the main draw as a lucky loser. There, he beat
Tomás Martín Etcheverry in the first round and lost to the 13th seed
Matteo Berrettini in the second round. Grenier broke into the top 100 of the
ATP singles rankings at world No. 95 on 12 September 2022, after winning the
Cassis Open Provence singles title, defeating
James Duckworth in the final.
2023: Masters debut and first clay-court wins, out of top 150 In March, Grenier won his fourth career
Challenger singles title in
Les Franqueses del Vallès, Spain. Grenier made his ATP Tour singles main draw clay-court debut at the
2023 Grand Prix Hassan II. There, he was defeated in the final qualifying round by
Dimitar Kuzmanov but entered the main draw as a
lucky loser, where coincidentally he lost again to Kuzmanov in the first round. Ranked No. 128 in the world, Grenier made his Masters 1000 singles main draw debut at the
2023 Mutua Madrid Open by winning two qualifying matches. In the main draw, he earned his first career ATP Tour singles main draw clay-court match win by defeating former top 10 player
Diego Schwartzman in straight sets in the first round. In the second round, he recorded his first career top 30 win by defeating 22nd seed
Sebastian Korda in a straight-sets match with two tiebreaks. In the third round, he lost to 13th seed
Alexander Zverev in straight sets 1–6, 0–6 in a 56-minute match. Ranked No. 124, he entered the next Masters 1000 in
Rome as a lucky loser but lost to
J. J. Wolf in the first round. Grenier made his
French Open singles main draw debut thanks to a wildcard, where he lost in the first round to the unseeded
Márton Fucsovics in four sets. In July, Grenier won his fifth career
Challenger singles title in
Pozablanco, Spain, defeating
Juan Pablo Ficovich in the final. In October, Grenier reached the final in
Alicante, Spain, where he lost to fellow countryman
Constant Lestienne in the final for the second time at this tournament, the first time being in 2021. Grenier finished the season outside the top 150 ranked at world No. 177. Next he also qualified for the
2024 Qatar ExxonMobil Open and defeated lucky loser
Maximilian Marterer. In March, Grenier qualified to the
Indian Wells Masters, his third career Masters appearance. In April, ranked No. 156, he also entered the main draw of the ATP 500, the
2024 Barcelona Open Banc Sabadell as a lucky loser for a second time at this tournament, after the late withdrawal of defending champion and top seed
Carlos Alcaraz. Ranked No. 173, he qualified for the
2024 US Open making his second appearance at this Major, losing to fellow qualifier
Mitchell Krueger in the first round.
2025: Back to top 200, Challenger final in two years In October, Grenier reached his first
Challenger final in two years at the
Villena Open, losing to third seed
Pablo Carreño Busta in the final. ==Performance timeline==