Will Armitage's first goal for Southampton's B team was cancelled out by a late Birmingham equaliser, as the spoils were shared on a frosty Friday in the Midlands.
Dom Ballard opened the scoring just after the half hour mark from the penalty spot as Saints managed the first half confidently.
Birmingham hit back in the second half before Armitage's header looked to have won the game in the 80th minute.
However, the hosts scored a stoppage-time header of their own to ensure the scores ended level.
Saints lined up for their penultimate Premier League Cup group game with more familiar faces restored from their previous outing.
Ollie Wright returned between the sticks, while other regulars Zuriel Otseh-Taiwo, Dom Ballard and Jimmy-Jay Morgan all started.
Dynel Simeu took his place at centre back, still eligible for B team fixtures for extra fitness alongside his loan at Tranmere Rovers.
In the sub-zero temperatures at Birmingham’s training facility, it was the hosts who worked the game's first half-chance after three minutes.
Experienced first team midfielder, Gary Gardner, hit the side netting with a sliding effort as the ball fell to him inside the area.
Saints fired back with their first opportunity in the 15th minute after carefully managing the opening stages; Ballard raced onto a Cam Bragg pass but dragged his shot wide.
The Saints forward almost turned provider just before the half hour mark with excellent hold up play in the area to find Morgan, who shot just wide of the post under pressure.
The visitors did make their growing pressure tell moments later, as a penalty was awarded for handball in the area.
Ballard confidently stepped up and buried his shot into the bottom left corner to notch his first goal in the competition.
Jack Turner then tried his luck with a free-kick from range that was gathered by Birmingham’s goalkeeper.
Just before the break, Wright was called into action to maintain Saints’ lead, denying Romelle Donovan’s angled drive with his feet.
As the second half resumed, the hosts came out searching for an equaliser with Wright needed again, this time smothering the ball from the feet of Rico Patterson who attempted to dribble past him after breaking through the back line.
Saints’ shot-stopper was alive again in the 68th minute, as a goalbound deflection out of nowhere had to be palmed away.
With the clock ticking into the final 20 minutes, the hosts’ search for an equaliser was finally rewarded.
Williams raced onto a loose ball that came from a midfield tackle and slotted home into the bottom corner.
Birmingham almost found their way into the lead moments later as Ryan Stirk’s low shot needed to be held by Wright.
Regaining their composure after being pegged back, Saints looked to restore their lead with Sam Bellis’ effort denied at the near post.
In the 80th minute the game swung once more as Armitage rose high from a corner to head home his first B team goal, much to the joy of his teammates.
The slender lead was almost increased in the closing stages as, barely a minute after coming on from the bench, Luke Pearce was denied by a double save inside the penalty area.
The late drama was unfortunately not over there, however, as the Blues had time for one final siege on goal.
Brandon Khela’s low shot was saved in the final minute of normal time, before a stoppage-time corner saw the scores levelled once more.
Rico Patterson met the cross with a powerful header that crashed in off the underside of the crossbar, earning a valuable point for the hosts.
Saints dropped their first points in the Premier League Cup as they settled for a draw, but remain unbeaten as group winners.