Birmingham held on to edge Watford 2-1 on Monday to move within a point of the Championship play-off places.
Midfielder Seung-ho Paik and winger Demarai Gray put Tom Brady's outfit ahead before the break at St Andrews.
Seven-time Super Bowl winner Brady owns a minority stake in Birmingham, and they were made to sweat in the second half after winger Othmane Maama's effort.
The Blues, League One winners last season, claimed a fourth straight home victory, to trail sixth-placed Bristol by the smallest of margins in the English second division's table.
South Korea's Paik opened the scoring after half an hour with a low strike, netting for a third time in five games.
Jamaica's Gray then doubled the Blues' lead two minutes before the break, with the hosts firmly on top.
Five minutes into the second-half Watford had a great chance to halve the deficit but Marc Bola had his effort tipped over the bar.
With half an hour left Bola played a part in bringing his side back into the game, setting up Morocco's Maamma to make it 2-1.
As Javi Gracia's side looked for the equaliser, he replaced Maamma with experienced France midfielder Moussa Sissoko, but the 36-year-old was unable to help his side find the crucial second goal, leaving the Hornets in 15th spot.