Even defeat to Bluewings would have left Pohang needing to lose their remaining three games and a huge swing in goal difference for their hosts to catch them and a defeat looked on the cards three minutes in at Suwon World Cup Stadium as Santos Junior gave hosts the lead.
However, a 33rd-minute effort from Lee Myung-Joo levelled the scores and Go Moo-Yul struck with 15 minutes to go to secure a top-four finish for Hwang Sun-Hong's side.
In the relegation round, Daejeon Citizen's remarkable recovery continued as they overcame Gangwon 3-1 on Saturday.
Citizen remain bottom of the table, but their third win in a row, just their fifth of the season, puts them within four points of Gangwon, who occupy the relegation play-off spot.
A Cesar Arias double, the latter from the penalty spot, was sandwiched by Choi Jin-Ho's strike, but Hwang Ji-Woong was on hand to seal the points and bring Gangwon's own five-game unbeaten run to an end.
Daegu remain between Citizen and Gangwon after slipping to a 1-0 defeat at home to Chunnam Dragons. Im Kyung-Hyun's 78th-minute goal was enough for Chunnam, who are on the brink of safety after snapping a six-game run without a win.
Ulsan continued their march to a third K-League title courtesy of Saturday's 2-0 triumph over Jeonbuk Motors.
Kim Ho-Gon's side were made to wait for their fifth win on the bounce but K-League top scorer Kim Shin-Wook and Caique struck in the final 10 minutes to maintain Ulsan's five-point advantage over Pohang.
Elsewhere, Busan I'Park had a 93rd-minute winner from Han Ji-Ho to thank as they beat Incheon United 2-1 while Jeju United beat Seongnam 1-0 in a battle of the relegation round's top two, both sides comfortably safe from the drop already.
