At the 2007 US Open, seeded tenth with Aspelin, Knowle achieved the greatest triumph of his career by winning the tournament, his first Grand Slam.

They went on to upset eighth seeds Jonathan Erlich and Andy Ram (who would go on to win the 2008 Australian Open men's doubles) in the third round.

In the semifinals, they held off unseeded Julien Benneteau and Nicolas Mahut, 7–6(2), 1–6, 6–3, before winning the final 7–5, 6–4 over the ninth seeds, Pavel Vízner and Lukáš Dlouhý.

The first Austrian to win a Grand Slam tournament was Thomas Muster at the 1995 French Open; the third was Jürgen Melzer, who won the 2010 Wimbledon Championships – Men's doubles and later the 2011 US Open – Men's doubles with his German partner Philipp Petzschner as well as the 2011 Wimbledon Championships - Mixed doubles with his later wife Iveta Benešová; the fourth was Dominic Thiem, who won the 2020 US Open - Men's Singles tournament.

Their excellent first year as a team enabled Knowle and Aspelin to participate in the Tennis Masters Cup in Shanghai, China for the first time.

They surprisingly made it all the way to the final, beating Pavel Vízner and Lukáš Dlouhý, Arnaud Clément and Michaël Llodra, and finally Martin Damm and Leander Paes, before eventually falling in straight sets to Mark Knowles and Daniel Nestor 2–6, 3–6.

They defeated the German duo of Nicolas Kiefer and Rainer Schüttler in three sets in the first round, before being knocked out of the tournament by Bob and Mike Bryan, 6–7(2), 4–6.

Knowle and Melzer enjoyed little success on the tour in the first half of 2009, before their performance improved significantly in the later weeks, winning titles in New Haven and Tokyo and reaching another final in Vienna.

His planned return to the tour failed, when a partially torn tendon prevented his participation in the French Open to defend his semifinal success from the previous year.

He reached the doubles final in Estoril with David Marrero and won the Kitzbühel tournament with Cermak, claiming his first title since Tokyo in 2009.

In April, Knowle won the Grand Prix Hassan II in Casablanca with Filip Polášek, winning the final over the German team of Dustin Brown and Christopher Kas.