All of the singles finals he has appeared in have come at the ITF Futures level with the exception of one, when he lost the 2012 The Hague Open challenger tournament in Scheveningen, Netherlands to Jerzy Janowicz of Poland in straight sets 2–6, 2–6.

Middelkoop made his ATP Tour singles debut in 2008 at the Ordina Open on the grass courts of 's-Hertogenbosch, Netherlands, when he was granted entry into the main draw as a lucky loser.

He entered the main draw as a lucky loser recipient after Finland's Jarkko Nieminen withdrew with a right heel injury, however he was promptly defeated by eventual runner-up Marc Gicquel of France again in straight sets 4–6, 1–6.

Middelkoop made his ATP Tour doubles debut at the 2008 ABN AMRO World Tennis Tournament held on hard courts in Rotterdam, Netherlands.

He was granted a direct acceptance into the doubles draw alongside compatriot Jesse Huta Galung, however they would be defeated in the first round by second seeds and eventual semi-finalists Julian Knowle and Simon Aspelin in straight sets 3–6, 5–7.

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

At the 2022 French Open he reached the semifinals for the first time at a Grand Slam in his career partnering Rohan Bopanna defeating en route the second seeds, former World No.