The following year he entered the qualifying draw of the same tournament, and defeated future ATP top 50 player Denis Istomin 6–0, 6–1 to win his first ITF junior match.

[4] Sitak partnered Polish tennis player Mateusz Kowalczyk to win the 2014 MercedesCup doubles title, defeating Philipp Oswald and Guillermo García López 2–6, 6–1, 10–7 in the final.

After that came the Davis Cup where, in Tianjin, Sitak and Marcus Daniell lost their doubles tie to the lowly-ranked Chinese pair of Gong Mao-Xin and Zhang Ze.

Sitak and Koolhof then lost in the quarter-finals 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.

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 6–3, 6–2, in the final, but they lost in the first round of their next tournament in Marrakech.

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

They had to come from two sets down in both their second and third round matches, against Julio Peralta and Horacio Zeballos and Marcin Matkowski and Jonathan Erlich respectively, before lining up against Mike Bryan and Jack Sock in the quarter-finals.

Sitak teamed up with Erlich for the Hall of Fame Championships in Newport, marching imperiously to the final, where they very quickly swept aside clay-court specialists Marcelo Arévalo and Miguel Ángel Reyes-Varela.

They beat Max Mirnyi and Philipp Oswald, then Marcelo Demoliner and Santiago González, before falling to Jamie Cerretani and Leander Paes in the semi-final.

They also lost in the quarter-finals in Auckland, this time to Bob and Mike Bryan, the former playing his first tournament since being injured in Monte Carlo and subsequently having surgery to replace his hip.

The Australian Open also saw them record a win and a loss, beating Leander Paes and Miguel Ángel Reyes-Varela in the first round, but losing a tight match to eventual semi-finalists Ryan Harrison and Sam Querrey in the second, 6–4, 7–6(5).

The same fate awaited them in Rio de Janeiro, but they had far better luck in Acapulco, where they finished runners-up to Mischa and Alexander Zverev after easily taking the first set of the final.

Because of their relative rankings, Sitak found a new partner for Indian Wells in singles specialist Nikoloz Basilashvili, but they went out in the first round to eighth seeds Henri Kontinen and John Peers.

They also got into the Italian Open as an alternate pair, and this time made it to the second round, losing to eventual champions Juan Sebastián Cabal and Robert Farah.

Sitak had different partners for each of his first three grass tournaments before winning the Antalya Open with Jonathan Erlich in a week where the on-court temperatures seldom dropped below 40 degrees Celsius.

They defeated Ivan Dodig and Filip Polášek in straight sets in the final, having beaten French Open champions Kevin Krawietz and Andreas Mies in the first round.

They were more successful in Auckland, upsetting top seeds John Peers and Michael Venus in the first round before losing a tight match in the second against Sander Gillé and Joran Vliegen.

Then followed three tournaments with a win and a loss – New York, Delray Beach (where they lost to eventual champions Bob and Mike Bryan) and the Chilean capital, Santiago.

Although he and Dominik Koepfer lost in the final qualifying round, they took their place in the main draw as Lucky Losers, but were beaten in a match tie-break by Marcelo Demoliner and Santiago González.

Sitak then headed to Europe, where he and Sergio Martos Gornés lost in the semi-finals of a Challenger event in Lugano, having won a marathon match-tie break (17–15) in their previous match.

At the 2021 Astana Open he reached the semifinal in doubles partnering Ričardas Berankis, where they lost to top seeds and eventual champions Santiago González and Andrés Molteni.

[10] Note: walkover victory when Pakistan abandoned the tie in 2013 is not counted as a match played Although the US and French Opens took place in 2020, mixed doubles were not included in either event due to the COVID-19 coronavirus.