Thornton qualified for the 2008 UK Open via the Pub qualifiers and then went on to switch to the PDC after withdrawing from the Scottish team for the Europe Cup, stating that his first goal was to get into the top 16 of the PDC Order of Merit which would guarantee his place in all the major tournaments. At the UK Open he lost 9–6 to Alan Tabern in the last 32. Thornton qualified for the Las Vegas Desert Classic losing again in the last 32 this time 6–5 against Adrian Lewis, but went one better at the World Grand Prix when he beat Peter Manley 2–1 in sets in the first round before being defeated 3–2 by Andy Hamilton in the last 16. His good start to his career in the PDC continued at the European Championship where he saw off John Part, Mensur Suljović and Ronnie Baxter to advance to the semi-finals for the first time and a meeting with Phil Taylor. Thornton averaged 102.12 in the match but Taylor averaged 113.33 as he won 11–7. Thornton's World Masters title from last year saw him qualify for the Grand Slam of Darts and he advanced from Group E with wins over Gary Mawson and Kirk Shepherd, before being edged out 10–8 by Terry Jenkins in the last 16. Away from the television cameras on the PDC Pro Tour he won the Australian Open Players Championship with a 3–1 success against Paul Nicholson. He was a losing semi-finalist three times and a quarter-finalist four times in the other events to finish as the highest non-qualified player on the Pro Tour Order of Merit for the 2009 PDC World Championship.