Harms qualified for the 2012 BDO World Darts Championship as the 13th seed. He defeated Martin Phillips, Robbie Green, and fellow débutant Paul Jennings to reach the semi-final where he was defeated 6–5 by Tony O'Shea. Harms's performance, followed by subsequent strong runs in the coming months, saw him briefly top the WDF world rankings. Among Harms's other achievements were reaching the semi-finals of the Winmau World Masters in 2012, repeating his world championship semi-final run in 2013 where he was once again beaten by O'Shea, winning the WDF World Cup in 2013 beating world number one Stephen Bunting 7–6 in the final, and reaching the semi-finals of the Zuiderduin Masters in 2013 where he was beaten 3–2 by Bunting. However, he was defeated in the second round of the 2014 World Championship by Tony Eccles after throwing for the match in the sixth set and missing one dart at double 16 to win, also leaving double 18 for the match in the next leg only to see Eccles hit 122 for the set.