Orange Democratic Movement's Imran Okoth is on the verge of clinching the Kibra parliamentary seat.
Preliminary results from 60 out of 183 polling stations show that the ODM candidate is leading with 8,515 votes.
Jubilee's McDonald Mariga is second with 2975 votes while Mr Eliud Owalo of Amani National Congress is third with 1,728. Mr Mariga, has however conceded defeat.
The contest had been considered by Deputy President William Ruto as a dry run for the 2022 General Election.
Dr Ruto is seeking to shore up his support base by making inroads in ODM leader Raila Odinga's turf.
While the DP has openly expressed interest in succeeding President Uhuru Kenyatta in 2022, Mr Odinga has not done so, saying it is still too early to start talking about 2022.
Earlier, ODM Secretary-General Edwin Sifuna exuded confidence that the Orange Party's candidate would win the seat.
"We have won the Kibra seat, as per our tally. We are only waiting for the official declaration of results and handing over of certificate to our candidate," Mr Sifuna said.