Tusker's seven-match winning streak in the Football Kenya Federation Premier League was halted after Bidco United defeated them 2-1 at Kasarani Annex on Saturday.
The hosts scored through Stephen Waruru and Batts Awita while the visitors struck courtesy of Humphrey Mieno.
With their current form, many expected the Ruaraka-based charges to start the game on a high and threaten regularly. But in contrast to that, it was the promoted side who started the game well.
Former Brewer Peter Nzuki had a chance to open the scoring in the sixth minute when the visitors conceded a free-kick in a good area, but the midfielder, uncharacteristically, blasted his effort wide.
The hosts did not have to wait long for an opening goal. Tusker conceded a free-kick in the 15th minute and David Kalama took it. The defenders failed to clear the ball and it fell at the feet of former Ulinzi Stars and Sofapaka FC striker Waruru, and the veteran forward had no problems in hitting the back of the net.
Ten minutes later, the 11-time champions had a chance to level matters when they won a free-kick on the edge of the 18-yard area, but Bidco custodian Omar Adisa was equal to the task.
Even after doubling their effort, especially after conceding, the Brewers were kept at bay and went to the break trailing by a solitary goal.
After the break, coach Robert Matano introduced Mieno, Erick Zakayo and Jackson Macharia in place of Hashim Sempala, Boniface Muchiri and Onyango.
Even after increasing their tempo, the Brewers failed to break down the resilient Bidco side.
With 10 minutes to go, defender Kevin Monyi was brought down in the penalty box and a penalty was awarded. Mieno held his nerve to hit the back of the net to ensure a tense finale with the scores level.
The goal did little to dampen Bidco's spirit and they, eventually, managed to grab the winner in the stoppage time courtesy of Awita.
The win took Bidco to seventh on the table with 20 points from 14 games while Tusker remain top with 32 points from as many matches.
Bidco United XI: Omar Adisa, David Kalama, Lambert Otieno, Collins Okumu, Francis Oduor, Zacharia Gathu, Jacob Onyango, Peter Nzuki, Alex Juma, Alex Juma, Erick Githimu, Stephen Waruru
Subs: Edwin Omwambani, Geoffrey Gitau, Wilson Anekeya, Anthony Simasi, Nelson Chieta, Batts Awita, David Orem, Hillary Shirao, Henry Omollo.
Tusker XI: Michael Wanjala, Kevin Monyi, Hillary Wandera, Christopher Oruchum, Vincent Ngesa, Hashim Sempala, Luke Namanda, Faraj Ominde, Henry Meja, Chrispinus Onyango, Boniface Muchiri.
Subs: Emery Mvuyekure, Jimmy Mbugua, Rodgers Aloro, Michael Idovolo, Humphrey Mieno, Erick Zakayo, Jackson Macharia, George Ogutu, Christopher Ochieng.