Bolton defender Radhi Jaidi salvaged a point for Tunisia with a powerful header in stoppage time in Munich.
Radhi Jaidi scored his sixth goal for Tunisia
Saudi Arabia's record goalscorer Sami Al Jaber looked to have given his side the win when he came off the bench to put them 2-1 up with six minutes to go.
Just 90 seconds after his introduction, veteran Al Jaber beat Tunisian keeper Ali Boumnijel with a left-footed shot.
Ziad Jaziri's first-half volley had given Tunisia the lead, before Yasser Al Kahtani equalised with a deft flick.
The draw means Spain are top of Group H with three points, with Tunisia and Saudi Arabia joint second on one point each.
Jaziri had served notice of the threat he posed with his quick feet and strong running as early as the second minute when he surged into the Saudi box.
The Tunisian striker went to ground under pressure from Omar Al Ghamdi and claimed a penalty, but Australian referee Mark Shield waved play on.
Jaziri took his goal superbly. With both feet off the ground he showed exquisite technique to lash a right-footed volley past Mabrouk Zaid.
If Jaziri's goal was exceptional, the defending by Saudi Arabia was rather less impressive, with Redha Tuker primarily at fault with a poor defensive header.
Tuker's header richocheted off Hamad Al Montashari and as the Saudi Arabian defence dithered, Jaziri pounched.
Tuker tried to make amends for his mistake with a well-taken free-kick that was comfortably saved by Boumnijel.
Tunisia made the fatal mistake of sitting on the lead, a tactical error they were punished for as Saudi Arabia adopted a bolder approach in the second half.
Key to the Saudis' revival was the influence of midfielder Mohammed Noor, who began to get forward into more advanced positions.
Soon after the restart Noor spurned a good chance to equalise when he flicked the ball wide with just Boumnijel to beat.
But Noor made amends for his miss when he delivered an enticing cross to the near past that Al Kahtani cleverly flicked into the net.
With 10 minutes to go Saudi Arabia introduced their record goalscorer Al Jaber for Al Kahtani for his 161st cap.
Al Jaber, who had an unhappy five-month spell at Wolves, did not take long to make his mark, calmly slotting past Boumnijel after galloping clear of the Tunisia defence.
Saudi Arabia almost extended their lead when captain Hussein Sulaimani's deflected free-kick crashed against the woodwork
But Jaidi, who has proved such a threat for Bolton with his strength, came to Tunisia's rescue, thumping a header past Zaid after Jaziri crossed from the right.
Tunisia: Boumnijel, Haggui, Trabelsi, Jaidi, Jemmali, Mnari, Bouazizi (Nafti 54), Chedli (Ghodhbane 69), Namouchi, Jaziri, Chikhaoui (Essediri 82).
Subs Not Used: Ayari, Ben Saada, Gmamdia, Kasraoui, Melliti, Nefzi, Saidi, Santos, Yahia.
Booked: Haggui, Bouazizi, Chedli, Chikhaoui.
Goals: Jaziri 23, Jaidi 90.
Saudi Arabia: Zaid, Dokhi, Tukar, Al Montashari, Sulimani, Al Ghamdi, Noor (Ameen 75), Khariri, Aziz, Al Temyat (Mouath 66), Al Kahtani (Al Jaber 82).
Subs Not Used: Al Anbar, Al Bahri, Al Daeyea, Al Harthi, Al Qadi, Al Shlhoub, Khathran, Khojah, Massad.
Goals: Al Kahtani 57, Al Jaber 84.
Ref: Mark Shield (Australia).
Fifa man of the match: Ziad Jaziri.
Tunisian goalkeeper Ali Boumnijel became the fifth oldest World Cup player at the age of 40 years and 62 days. His teammate Yassine Chikhaoui became the youngest player to appear in a World Cup match for Tunisia, at the age of 19 years and 265 days.
Tunisia have now failed to win nine consecutive World Cup matches, the worst current run by a 2006 World Cup participant. Saudi Arabia have now extended their winless streak to eight World Cup matches.
Al Kahtani became the first Saudi player to score from open play since Fahed Al-Ghesheyan at the 1994 World Cup. Sami Al Jaber scored the second goal just two minutes after he came on as a substitute. The record for the fastest goal by a substitute is 18 seconds by Uruguay's Richard Morales against Senegal at the 2002 World Cup.