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 straight sets before losing to fifth seed German Jan-Lennard Struff by walkover in the second round.^[2] In November, Grenier won his first career ATP Challenger Tour singles title in Roanne, France.

2022: Grand Slam and top 100 debut

He made his Grand Slam singles main draw debut 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.^[3] 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.^[4]

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.^[5]

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.^[6] 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.^[7][8] 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.^[9] In the third round, he lost to 13th seed Alexander Zverev in straight sets 1–6, 0–6 in a 56-minute match.^[10] 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.^[11]

In July, Grenier won his fifth career Challenger singles title in Pozablanco, Spain, defeating Juan Pablo Ficovich in the final.^[12] 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. ^[13]

2024: Australian Open debut and first win

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.

Singles

Singles: 22 (11–11)

Result	W–L	Date	Tournament	Tier	Surface	Opponent	Score
Loss	0–1	Jul 2015	Kuwait F1, Mishref	Futures	Hard	Lennert Van der Linden	4–6, 6–7(1–7)
Loss	0–2	Mar 2016	Italy F2, Basiglio	Futures	Hard (i)	Antal Van der Duim	6–7(2–7), 2–6
Loss	0–3	Jul 2016	Portugal F8, Idanha-a-Nova	Futures	Hard	Joao Monteiro	3–6, 2–6
Loss	0–4	Aug 2016	Italy F26, Piombino	Futures	Hard	Matteo Trevisan	5–7, 4–6
Win	1–4	Jan 2017	France F3, Veigy-Foncenex	Futures	Carpet (i)	Clement Geens	6–3, 6–4
Loss	1–5	Feb 2017	Great Britain F1, Glasgow	Futures	Hard (i)	Filip Horansky	2–6, 3–6
Loss	1–6	Jul 2017	Israel F12, Tel Aviv	Futures	Hard	Antoine Escoffier	7–6(7–4), 4–6, 2–4 Ret.
Loss	1–7	Aug 2017	Belarus F3, Minsk	Futures	Hard	Dzmitry Zhyrmont	6–7(4–7), 2–6
Loss	1–8	Jul 2018	Spain F17, Bakio	Futures	Hard	Roberto Ortega Olmedo	2–6, 0–3 Ret.
Win	2–8	Mar 2019	M15 Toulouse, France	World Tennis Tour	Hard (i)	Raphael Baltensperger	4–6, 6–3, 6–4
Win	3–8	Apr 2019	M15 Cancún, Mexico	World Tennis Tour	Hard	Lorenzo Frigerio	2–6, 7–6(7–3), 6–2
Win	4–8	Apr 2019	M15 Cancún, Mexico	World Tennis Tour	Hard	Gage Brymer	6–3, 6–2
Loss	4–9	Jul 2019	M25 Bakio, Spain	World Tennis Tour	Hard	Laurent Lokoli	3–6, 3–6
Win	5–9	Nov 2019	M25 Saint-Dizier, France	World Tennis Tour	Hard (i)	Harold Mayot	6–1, 7–5
Win	6–9	Jul 2021	M25 Bakio, Spain	World Tennis Tour	Hard	Adrián Menéndez Maceiras	6–0, 6–1
Loss	6–10	Oct 2021	Alicante, Spain	Challenger	Hard	Constant Lestienne	4–6, 3–6
Win	7–10	Nov 2021	Roanne, France	Challenger	Hard	Hiroki Moriya	6–2, 6–3
Win	8–10	Jul 2022	Segovia, Spain	Challenger	Hard	Constant Lestienne	7–5, 6–3
Win	9–10	Sep 2022	Cassis, France	Challenger	Hard	James Duckworth	7–5, 6–4
Win	10–10	Mar 2023	Les Franqueses del Vallès, Spain	Challenger	Hard	Billy Harris	3–6, 6–1, 7–6(7–3)
Win	11–10	Jul 2023	Pozoblanco, Spain	Challenger	Hard	Juan Pablo Ficovich	6–7(4–7), 6–2, 7–6(7–3)
Loss	11–11	Oct 2023	Alicante, Spain	Challenger	Hard	Constant Lestienne	7–6(12–10), 2–6, 4–6