Mistakes by goalkeeper Kennedy Mweene – who gifted the Buccaneers their goal and then missed a second half penalty – had threatened to hand Pirates all three points, but Tsotetsi popped up in the third minute of injury time to deny the visitors a second straight win in the league.
Pirates took the lead just nine minutes into the game thanks to a freak goal from Dikgang Mabalane. The former Cosmos winger showed up Mweene's poor positioning with a high, swirling free-kick from the right flank that drifted beyond the reach of the Zambian and hit the inside of the post before crossing the goal line.
Sthembiso Ngcobo and Matome Madimabe looked to be Stars' best players in possession and the latter came close to bringing his side on level terms with Pirates in the 29th minute, striking an imaginative effort with the outside of his right foot that flashed just wide of Moeneeb Josephs' right hand post.
Ten minutes later Madimabe tested Josephs with a curling left-footed effort, but the goalkeeper made a comfortable save, before berating his defence for allowing the playmaker enough time and space in which to line up a shot.
Mweene, at fault for the goal in the first half, almost gifted Pirates a second five minutes into the second when he dropped the ball under pressure from a corner kick, but the visitors were unable to capitalise on the situation.
Ea Lla Koto, meanwhile, upped their efforts to get back on terms and coach Themba Sithole bravely used all his substitutes before an hour of the game had even lapsed, although the removal of Thegofatso Mashego and Madimabe was due more to niggling injuries than tactics.
On the hour mark, former Stars player Thulasizwe Mbuyane set up Gilbert Mushangazhike for a difficult chance at the near post, and the Zimbabwean almost beat Mweene with a glancing touch that took the ball across the face of goal.
One of Stars' best chances to draw level saw striker Diyo Sibisi power his way past Lucas Thwala in the 73rd minute and have a clear sight of goal, but Josephs made a sharp save at his near post, turning the ball behind for a corner kick.
Three minutes later the Buccaneers should have made it 2-0, but Thembile Kanono directed his shot wide of goal after being unselfishly set up by Mbuyane.
Pirates' profligacy should have cost them two points, as almost immediately the hosts were correctly awarded a penalty after Rooi Mahamutsa and Thwala combined to bundle over Sibisi.
However, under intense intimidation from Josephs, Mweene blasted the spot kick high and wide, leaving Pirates' 1-0 lead intact.
The Stars 'keeper added to the day's drama when his mistake on the ball allowed Lebohang Mokoena through on goal, but the Zambian virtually rugby tackled the young forward to the ground to deny him a scoring chance, giving away another penalty in the 83rd minute. Mweene also received a yellow card for the foul, though it could just as easily have been red.
The goal minder redeemed himself somewhat by saving Lehlohonolo Seema's spot kick, and Tsotetsi spared his blushes with a towering header deep into injury time, meeting Ruben Cloete's great cross and powering an effort into the top corner.