Winstanley secured narrow 3–2 and 4–3 wins over Wayne Jones and Darren Webster in the 2015 World Championship to reach the third round of the event for the first time. He dropped the first two sets without winning a leg against Vincent van der Voort, but improved to trail 3–2. Winstanley missed one dart to take the tie into a final set and was instead defeated 4–2. He lost 5–3 to William O'Connor in the second round of the UK Open. Winstanley did not qualify for the rest of the PDC majors in 2015 and could not get past the last 16 in any Pro Tour event.