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.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.