The S.League title race was blown wide open when leaders Albirex Niigata (S) suffered a surprise 2-1 defeat at Hougang United while Home United beat Balestier Khalsa 2-1 on Monday night.
It was a second successive defeat for the champions to leave them just two points ahead of Home United after 13 matches of the 2017 season.
Albirex had led through Ryota Nakai's early penalty. But a weakened Hougang side, with eight changes from last Friday's 3-2 win against Home United, turned the game around via goals from Pablo Rodriguez and Fairoz Hasan.
Rodriguez was eager to make an impression after being dropped for the last six games. And he almost scored on six minutes, firing over from close range after a neat one-two with Fairoz Hasan.
Albirex, however, broke the deadlock four minutes later. Nakai converted from 12 yards out after being felled by Atsushi Shirota in the box.
Hougang could have levelled matters right after the restart, but Fairoz's snapshot from 20 yards out came off the crossbar.
Rodriguez then went close again on 25 minutes, with his header from Faiz Salleh's cross tipped over the bar by Albirex custodian Yosuke Nozawa.
The Spaniard's efforts finally paid off a minute later, as he connected well with Zulfahmi Arifin's inswinging corner to nod the Cheetahs level with his first competitive goal for the club.
Hougang stand-in goalkeeper Ridhuan Barudin then ensured both sides went into the break level. He denied Albirex captain Shuto Inaba in a one-on-one opportunity in the 43rd minute.
The Cheetahs scored within two minutes of the restart to go 2-1 up.
Gareth Low's deep cross from the left caused a defensive mix-up between Rui Kumada and Naofumi Tanaka. It led to Fairoz stealing in to poke home the loose ball from six yards out for his first goal since July 2015.
Albirex dominated the final half-hour, but were unable to break down a stout Hougang defence, led by White Swans' old boy Atsushi Shirota.
They almost grabbed an equaliser four minutes into added time. Shoichiro Sakamoto wriggled his way through to goal, but Ridhuan was on hand to thwart the Albirex danger from point-blank range.
That sealed a shock win for Hougang to help them climb above Geylang to go fifth, with 19 points from 12 matches.
Both teams meet again four days later at the same venue in the Singapore League Cup group stages.
Hougang United: Ridhuan Barudin (GK, C), Faiz Salleh, Gerald Ting, Atsushi Shirota, Lionel Tan, Gareth Low (Nazrul Nazari 83'), Afiq Noor (Fabian Kwok 71'), Zulfahmi Arifin, Fairoz Hasan (Justin Hui 64'), Antoine Viterale, Pablo Rodriguez
Albirex Niigata (S): Yosuke Nozawa (GK), Hiroyoshi Kamata, Rui Kumada, Takuya Akiyama, Naofumi Tanaka, Shoichiro Sakamato, Shuto Inaba (C), Kento Nagasaki, Koki Sato, Ryota Nakai, Tsubasa Sano