Defoe was in danger of missing the game through tiredness, but if he was exhausted then Birmingham looked unconscious as the England star scored a scorcher to wake this game from its slumber.
Little of note had happened in the 37 minutes before Defoe decided to take things into his own hands.
The 21-year-old collected Noe Pamarot's pass 40 yards out, skipped past two challenges before beating Kenny Cunningham and unleashing a vicious 25-yard shot beyond Maik Taylor.
Defoe, who is hoping to be called up for England's World Cup qualifiers against Austria and Poland, could not have timed his third goal of the season any better.
He caused more panic in the Birmingham defence before half-time with another 20-yard shot and then did some more fancy dribbling, before being mysteriously booked by referee Mark Clattenburg for diving after he appeared to have been brought down by Cunningham on the edge of the box.
The home side, who went into the match unbeaten after three games, went in at the break in better spirits than expected.
Tottenham fans had been forced to be satisfied with only seeing new £3million signing Michael Carrick on the pitch before the game, as boss Jacques Santini ruled he was not fit enough to make his debut.
But they were boosted by the return of striker Robbie Keane to the starting line-up for the first time this season after an ankle injury.
The duo's presence was the only thing the home support had to cheer about in a dismal first half hour which saw plenty of effort but little craft.
Both sides could only boast five goals between them from their opening three games and it was easy to see why.
Neither team could keep the ball for more than a few seconds and nor were there many moments to get supporters off their seats.
Pedro Mendes let fly with a couple of speculative efforts from a good 35 yards out which did not trouble keeper Maik Taylor in the slightest.
Keane then struck a fierce low drive from the edge of the box and it narrowly missed the left hand post.
Birmingham, who have failed to win a league game away from home in 2004, were even less productive.
The closest they came was when Matthew Upson pumped a free-kick to the back post and Jesper Gronkjaer headed the ball down to Mikael Forssell, but the striker's shot was well blocked by Noureddine Naybet.
There was little sign of improvement from the visitors after the break as they struggled to break the Spurs defence down.
Tottenham only conceded two goals in their first three games and Birmingham had yet to register a shot on target.
But they began to push more players forward and a few gaps appeared as Forssell came close to scoring his first of the season.
Emile Heskey did brilliantly to rob Mendes of possession and set Julian Gray free on the left. The winger found Forssell in space in the area but the Finnish international sent his shot over the bar.
It gave the visitors more confidence however and on 65 minutes Tottenham were thankful that Michael Brown was awake at the near post.
Heskey climbed highest to head Gronkjaer's corner towards goal, but Brown was there to clear the ball off the line.
Gray then sent a volley over the bar and an air of anxiety began to creep around the White Hart Lane stands.
Tottenham were providing very little attacking threat and Santini replaced Keane with Frederic Kanoute in a bid to improve matters.
Birmingham continued to have the better of the match, but Heskey's left-footed shot was met by jeers as it flew high into the stands.
There was still time for Paul Robinson to earn his wages by stopping Johnnie Jackson's shot in the closing stages, but Heskey could have had the final say.
Heskey was brought down by Robinson in the box, but was booked for diving and then in the dying seconds headed just over the bar.
Man of the Match - Jermain DefoeWhile all others struggled around him, Defoe provided the magic and the goal that made the admission fee worthwhile.