Mathare United fell 2-1 in their Football Kenya Federation Premier League match away to Bidco United at Kasarani Annex on Saturday.
Tyson Otieno scored for the Slum Boys while the hosts replied through Erick Gichimu and Lennox Ogutu, who scored an own goal.
It was the 2008 champions who started the match better hoping to get an early goal. They were rewarded in the seventh minute when James Kinyanjui picked out the run of Otieno who managed to convert the chance.
In the 20th minute, the Thika-based charges were forced to an early change when Kassim Mwinyi sustained a head injury and Anthony Simasi took his place.
The promoted side settled well and tried to give their best in search of an equalizer but failed to break their opponents' defence.
The Slum Boys had chances to score but Daniel Otieno and Danson Kago failed to hit the target.
Bidco had to wait until the 75th minute to come to level terms through Gichimu, whose header hit the crossbar before bouncing into the net.
With a minute to go, Ogutu played a backpass to his goalkeeper who was out of position and it turned out to be an own goal.
In another league assignment, 10-man KCB suffered a 2-1 loss away to Nzoia Sugar.
The Zedekiah Otieno-led charges scored first in the 10th minute. The Sugar Millers conceded a penalty and captain Simon Munala rose to the occasion and sent the goalkeeper the wrong way.
Gabriel Wandera replied in the 34th minute through a penalty as well. It got worse for the Nairobi-based charges in the 41st minute when Michale Mutinda was sent off.
Three minutes after the restart, Kevin Juma scored the second for his team which won maximum points for his charges.
At Kisumu Stadium, Western Stima also fell 3-2 to Kakamega Homeboyz.
Sylvester Owino scored first in favour of the visitors in the 17th minute before Kennedy Otieno replied for the hosts two minutes later.
The Powermen doubled their advantage in the 31st minute through Isaac Mweru.
In the 83rd minute, Clinton Bulimo was sent off and Homeboyz capitalized on the numerical advantage to equalise in the 85th minute courtesy of Robert Arrot.
The winning goal came in stoppage time through veteran striker Allan Wanga.
Bidco United XI: Adisa Omar(GK), David Kalama, Lambert Otieno, Francis Oduor(C), Mwinyi Kassimu, Gathu Zacharia, Jacob Onyango, Batts Awita, Alex Juma, Alex Juma, Eric Gichimu, Stephen Waruru
Subs: Edwin Omwabani(GK), Geoffrey Gitau, Wilson Anekeya, Anthony Simasi, Nelson Chieta, Hillary Shilaho, Nelson Chieta, Hillary Shilaho, David Orem, Peter Nzuki
Mathare United XI: Job Ochieng, Martin Ongori, Martin Werunga, Lennox Ogutu (C), Norman Ogolla, Cersidy Okeyo, Tyson Otieno, Danson Kago, James Kinyanjui, Daniel Otieno, Klinsmann Omulanga
Subs: David Okello, Dennis Ochieng, Anthony Otieno, Chris Oduor, Alphonse Ndonye, Boniface Akenga, Khalid Jumaan, John Mwangi, Dani Lual