The American lost his second match in 2021 but then won five in a row to lift the trophy and won his first two in 2022 to make it seven on the spin and take control of Group 12.
It looked like Pieters was going to secure a play-off at 2 Up with two to play but Horschel birdied the 17th and 18th to advance to the knockout stages.
"Thomas came to play today and he played a beautiful round of golf," said Horschel. "So when your opponent's not gifting you anything, then you've got to be on your game.
"I hung in there. I hit some really quality shots. Some of them didn't turn out. I made some putts, which was nice to see.
"Being two down with two to go and knowing all I needed was a half to move on to round of 16, that's really a confidence boost to birdie the last two to get through."
Pieters birdied the second and third but Horschel picked up shots on the fifth and ninth to make it level at the turn.
A Horschel bogey on the 13th and a Thomas birdie on the 16th put the 2016 Ryder Cup star 2 Up but Horschel put his tee-shot on the 17th to 13 feet and then put his second to three feet on the last to advance.
Min Woo Lee and Tom Hoge halved the other match in the group.
Horschel will next face Scottie Scheffler - the man he beat in last year's final - after the American beat Matt Fitzpatrick in a play-off.
Scheffler thumped Englishman Fitzpatrick 5 & 4 earlier in the day to set up the play-off but the extra holes were a much tighter affair.
The duo sandwiched three pars with a pair of birdies over the first five holes but Scheffler played an excellent up-and-down on the sixth to birdie the par five and advance from Group Five.
Tyrrell Hatton defeated Daniel Berger 2 Up to advance out of Group 13 with a 100 per cent record.
The Englishman has now made it out of the group stages three times in five appearances at Austin Country Club.
He birdied the third and fifth to hit back after Berger had won the second and fourth and while a bogey was enough to take the lead on the eighth, he made a double on the next to level things up at the turn.
A par edged him ahead on the 12th but Berger birdied the 14th before Hatton made gains on the 16th and 18th.
"It was a hell of a pillow fight," he said. "Neither of us played great. To be fair, I feel like if that finished in a half you'd have probably say it was fair. It wasn't great golf from either of us.
"Hoping that I can kind of go a little bit further than I have done. When I have made the weekends, I've lost on Saturday mornings."
Hatton will next face Seamus Power, who advanced despite a 2 &1 loss to Keith Mitchell.
The Irishman entered the day with two wins in Group Four and a 2 & 1 victory for the already-eliminated Patrick Cantlay against Sungjae Im ensured he would not be caught.
"I didn't play my best today," said Power. "I played probably the worst I've played. It was a weird day obviously because not winning but still going through.
"Obviously you want to get to knockout and now it kind of actually restarts to a certain extent. Now it's full knockout. It's all new to me so it's going to be exciting. I'm looking forward to tomorrow."