Scotland’s Connor Syme carded an incredible bogey-free 63 to take a one-shot lead heading into the final round of the Celtic Classic in Wales.
However, Pieters stumbled and finished the round in seventh place at one-under-par.
Horsfield remains Syme’s closest challenger, recording a 68 round, one-shot clear of Sebastian Soderberg.
Syme’s last win came at the Turkish Airlines Challenge in 2019 and will be looking to secure his first ever European Tour win on Sunday after coming second at the 2018 Shot Clock Masters.
The Celtic Classic is the fourth event of the six-week U.K. Swing, taking place in an isolated environment, after the tour resumed following a five-month stoppage because of the coronavirus outbreak.