PRAGUE, 29 Aug 2015, (AFP) - The 23-year-old, who attended the University of Illinois, is chasing his first professional victory with Sweden's overnight leader Pelle Edberg one shot back.
Denmark's Thorbjorn Olesen is alone in third on 13-under and four back with English pair Matthew Fitzpatrick and Robert Dinwiddie a further stroke adrift.
Pieters could have taken a stronger lead into the final 18 holes had he not double-bogeyed the 11th hole after playing the wrong ball, but still signed for an impressive round of seven under par.
He began his third round in sparkling fashion with an eagle at the first before birdieing the second to lie three under after just two holes.
"The 11th hurts a little bit because it is my mistake. It could have been a lower one but I'm still really happy with the way I played, and seven under is a good score on a Saturday," said Pieters.
"I was hitting it well and putting well. There was no reason to get too upset. Things happen, it's just a shame it happened today.
"It was nice to fight back and finish off with a couple of birdies, but I need to learn from it obviously and not make stupid mistakes like that.
"I was a little impatient in Spain when I was up there, especially the first six holes, so I need to stay patient tomorrow (Sunday)," he added in reference to a runner-up spot at the Spanish Open in 2014.
"There will always been some nerves but I've been there before in Spain and I just need to stay as calm and patient as I can."
Leading scores after the third round of the EPGA Czech Masters on Sunday at the Albatross Golf Resort (GBR and IRL unless stated - par 72)
199 - Thomas Pieters (BEL) 66 68 65
200 - Pelle Edberg (SWE) 66 67 67
203 - Thorbjorn Olesen (DEN) 67 69 67
204 - Robert Dinwiddie 69 67 68, Matthew Fitzpatrick 66 67 71
207 - Sam Hutsby 66 70 71, Mikael Lundberg (SWE) 72 71 64, Roope Kakko (FIN) 67 72 68, Peter Uihlein (USA) 70 71 66
208 - Craig Lee 68 71 69, David Lipsky (USA) 71 69 68, Eddie Pepperell 69 68 71, Michael Hoey 67 70 71, Renato Paratore (ITA) 65 73 70, Florian Fritsch (GER) 69 70 69, Jose Manuel Lara (ESP) 71 68 69
209 - Anthony Wall 72 70 67, Lasse Jensen (DEN) 71 72 66, Kristoffer Broberg (SWE) 67 73 69, Ryan Fox, Rikard Karlberg (SWE) 70 68 71, Jaakko Makitalo (FIN) 71 70 68, Pedro Orial (ESP) 67 73 69