1 for the first time in November 2022, becoming the fourth Dutchman to attain the top ranking in doubles after Tom Okker, Paul Haarhuis and Jacco Eltingh, and ending the year as joint No.

Koolhof won his maiden ATP tour doubles title at the 2016 Sofia Open on indoor hardcourts in Bulgaria, partnering compatriot Matwe Middelkoop.

The pair defeated Adil Shamasdin and Philipp Oswald in a third set tie-breaker 5–7, 7–6(11–9), [10–6] in the championship match to capture the title.

Sitak and Koolhof then went to Newport Beach in California where, as top seeds, they lost in the first round in straight sets to Treat Huey and Denis Kudla.

They then lost in the quarterfinals at Montpellier before going all the way to the final in the New York Open, being beaten by Max Mirnyi and Philipp Oswald in a match tie-break.

Their up and down season continued in Brazil, where they reached the final in São Paulo, but were beaten in straight sets by Federico Delbonis and Máximo González.

Moving to Europe, and playing in the Alicante Challenger in Spain as preparation for the European clay court season, they won their second title together when they beat Guido Andreozzi and Ariel Behar in the final in straight sets.

In the Hungarian Open they beat the top seeds Nikola Mektic and Alexander Peya in the first round, but lost in the second to Marcin Matkowski and Sitak's former partner Nicholas Monroe.

Seeded seventh, they reached their second Masters 1000 final at the 2022 Mutua Madrid Open after defeating John Isner and Hubert Hurkacz.

[6] In the final they defeated fifth seeds Robert Farah and Juan Sebastián Cabal to win their first Masters 1000 title in their career and as a pair.

[8] He also reached the quarterfinals with Skupski for the second time at this Major defeating unseeded pair of Americans Tommy Paul and Mackenzie McDonald.

4 on 8 August at the start of the 2022 National Bank Open where he reached with Skupski the semifinals of a Masters 1000 for the third time in the season defeating Lloyd Glasspool/Harri Heliövaara.

[15] At the 2022 Rolex Paris Masters the pair Koolhof/Skupski reached the semifinals defeating ninth seeds Rohan Bopanna/Matwe Middelkoop climbing to World No.

1 duo Koolhof and Skupski continued their good form reaching yet another Masters final at the 2023 BNP Paribas Open where they lost to Rohan Bopanna and Matthew Ebden.

[23] In 2024, after reuniting with Nikola Mektić for his last season, he won his first ATP 500 title at the home tournament, the 2024 ABN AMRO Open in Rotterdam.

[24][25] Unseeded, he won the 2024 BNP Paribas Open with Mektic defeating Marcel Granollers and Horacio Zeballos in the final.

[29] Koolhof and Mektić won their fifth title of the season at the Paris Masters at Bercy, defeating Lloyd Glasspool and Adam Pavlásek in the final which went to a deciding champions tiebreak.

[31] Partnering Botic van de Zandschulp Koolhof won the decisive match and helped the Netherlands team reach the semifinals at the 2024 Davis Cup, with a win over Spanish duo Carlos Alcaraz and Marcel Granollers.