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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. On 12 September, he broke into the top 100 of the ATP singles rankings at world No. 95 after winning the Cassis Open Provence singles title, defeating James Duckworth in the final.

2023: Masters 1000 debut, 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.

2024: Australian Open debut and first win

Ranked No. 178, he qualified for the 2024 Australian Open defeating compatriot Giovanni Mpetshi Perricard making his debut at this Major and recorded his first ever win at the tournament over another Frenchman Alexandre Muller.

He qualified for the 2024 Open 13 Provence but lost to Sebastian Korda. 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.