Tottenham manager Redknapp was branded 'Judas' by Pompey fans after quitting to take charge at White Hart Lane last October.
Portsmouth drafted in extra stewards to protect the Tottenham dugout but Redknapp was given a surprisingly muted reception as he emerged from the tunnel prior to kick-off.
Defoe came within inches of giving Spurs the lead in the ninth minute, racing in behind the Portsmouth defence on to Tom Huddlestone's threaded pass before rifling a right-foot shot against the left-hand post.
Portsmouth should have been in front two minutes later when striker Aruna Dindane produced a contender for miss of the season.
Tommy Smith cleverly turned Vedran Corluka down the right touchline before whipping in a pin-point cross only for Dindane to blaze over the crossbar from three yards with the goal at his mercy.
Portsmouth were denied by a wonder save from Heurelho Gomes in the 23rd minute. Younes Kaboul's driven free-kick struck Smith's heels and looped up towards the top right corner before Gomes flicked up a despairing left arm to turn the ball behind for a corner.
Tottenham owed a huge debt of gratitude to Portsmouth goalkeeper David James when they went ahead in the 29th minute.
Niko Kranjcar's corner brought the England international racing out of his goal but James got nowhere near the ball and Ledley King had the simple task of heading into an empty net.
Defoe, who was booed every time he touched the ball, rammed the Pompey fans' taunts down their throats by doubling Tottenham's lead on the stroke of half-time.
Jermaine Jenas was given too much freedom on the edge of the 18-yard box and Defoe slid in to turn his centre past a stranded James before wheeling away to celebrate in front of the home supporters.
The match was turned on its head by two incidents in the space of two minutes at the start of the second half.
First, in the 59th minute, Kevin Prince-Boateng reduced the arrears, firing past Gomes at his near post with a sublime half-volley from Michael Brown's cross.
But Defoe was then shown a straight red card for stamping on Pompey captain Mokeona after the South African had slid in for a challenge.
The home side could not take full advantage and force an equaliser and were also reduced to ten men late on when Brown was shown a second yellow card for a foul on Aaron Lennon.