However, Klein has struggled to make the transition onto the men's professional circuit, failing to enter the top 150 or gain entry into any Grand Slam event without being given a Wild card.

Klein began his professional tennis career playing on the ITF Men's Circuit in various events across Australia aged just 15.

[1] He featured sporadically on the ITF Circuit throughout 2005 and 2006, winning his first title alongside Matthew Ebden in a doubles event in Traralgon in October 2006.

[1] At the start of 2007, Klein competed in the Australian Open Junior Championships, and won the event after beating Jonathan Eysseric in three sets in the final.

[2] Throughout 2007, Klein began playing Futures tournaments on a much more regular basis, and secured his first singles title in Sawtell, Australia in October that year, beating Miles Armstrong 6–1, 6–3 in the final.

Shortly after, he returned to Futures action and earned his second singles title win in Mildura, defeating number one seed Nathan Healey in the final.

[3] A first-round loser in Brisbane, Klein acquired his first Grand Slam victory at the Australian Open against Björn Phau 6–4, 6–3, 4–6, 6–3, before falling to Stanislas Wawrinka in straight sets in the second round.

[3] Following his Australian Open efforts, Klein won the 2009 McDonald's Burnie International, the first challenger title win of his career.

[6] Klein began to play at Challenger level and on the ATP World Tour throughout 2009 to little success, although did reach a career singles high of no.

[3] Klein subsequently returned to the Futures Circuit in an attempt regain form, and was able to secure his first singles title in over a year when he defeated compatriot John Millman in three sets in Little Rock, Arkansas in April 2010.

[3] He struggled for form throughout the year, and competed at Futures level for the remainder of his 2011 campaign, yielding just one singles title in Kalgoorlie, Australia, in October.

[1] He spent the summer months of the year playing in Turkey and Spain Futures events;[1] beating Frenchman Jules Marie to win in Bakio, Spain, as well as securing two titles in Turkey by beating the young Italian Lorenzo Giustino in the final in Tekirdağ and then Mohamed Safwat in Antalya.

[1] He also won consecutive Challenger doubles titles alongside compatriot Dane Propoggia; firstly in San Benedetto del Tronto and then in Recanati.

[15][16] Klein's fine form during the summer continued when he reached the semi-finals of the 2012 President's Cup in Astana, Kazakhstan, before losing to the eventual champion, Evgeny Donskoy.

[1] Klein began the year by competing at the 2013 Australian Open, but lost in straight sets in the first round of qualifying to Riccardo Ghedin.

[19] The pair would go one better in early March 2013, securing their third Challenger title by beating Alex Bolt and Nick Kyrgios in the final of the 2013 Nature's Way Sydney Tennis International.

"[24] The move also allowed Klein to "ease the financial burden" that has meant he had to fund the previous three years of touring himself, only gaining access to a coach when he was competing in Turkey.

[13][29] Talking about the incident in December 2010, Klein's father said[30] — "It is sad to see him continually punished for what was, in truth, one single word muttered inaudibly over 18 months ago.