But it proved a step too far for the unseeded pairing as Mirnyi and Oswald quickly took control to capture the title.
Despite the loss the 32-year-old Kiwi is likely to improve from his current ranking of 59 to just outside the top 50.
Sitak and Koolhof will now head to Delray Beach for for this week's ATP tournament. They will play American Scott Lipsky and Indian Divij Sharan in the first round.
Michael Venus ranked 17th and his South African doubles partner Raven Klaasen are top seeds for the first ATP250 tournament in Marseille. They will face the Russian pair of Danil Medvedev and Mikhail Elgin in the first round.
Marcus Daniell and his British partner Dominic Inglot are the four seeds and have been drawn against Germany's Maximillian Marterer and Portugal's Joao Sousa in the first round.
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