Inter lost 3-1 at Catania on Friday and second-placed Milan looked to have wasted their chance to put huge pressure on the stuttering leaders until the Dutchman found the top corner from the edge of the box.
Mid-table Chievo hit the post and had a goal debatably ruled out for offside and Milan's David Beckham suffered a suspected ruptured Achilles tendon which threatens his World Cup with England.
"I'm very happy because tonight was tough. Inter know we are only a point away and we'll fight for the championship until the end," Seedorf told Sky TV after Milan got over their Champions League elimination by Manchester United in midweek.
"Inter's loss allowed us to quickly forget the Champions League defeat," added a jubilant Seedorf, who has not always been a favourite of the San Siro crowd.
Inter have won just one league game in six and held a nine-point lead in mid-February.
Third-placed AS Roma were held 3-3 at Livorno amid a flurry goals in the afternoon kick-offs, which summed up the poor standard of Serie A rather than its great excitement.
A Cristiano Lucarelli hat-trick for third-bottom Livorno meant Roma could only move to within six points of champions Inter with 10 games left.
Simone Perrotta and Luca Toni helped put Claudio Ranieri's side 2-1 up and David Pizarro made it 3-2 before the Chilean hit the post with a penalty and Lucarelli levelled the scores from the spot on 71 minutes after Juan's handball.
Palermo stayed fourth despite a 3-2 defeat at Udinese, who went seven clear of the drop zone largely thanks to goals either side of the break from striker Antonio Floro Flores.
HAPLESS GRYGERA
Juventus, enduring a nightmare season, could have gone above the Sicilians into the fourth and final Champions League qualifying spot but blew a three-goal lead to draw 3-3 with bottom side Siena in Turin.
Alessandro Del Piero scored his 300th and 301st career goals inside the first seven minutes with a close-range finish and superb volley before Antonio Candreva thumped in a third.
Siena hit back through Massimo Maccarone and Abdelkader Ghezzal, who then netted a leveller from the spot on 74 minutes after hapless Juve defender Zdenek Grygera fouled Maccarone.
Sampdoria are a point behind fifth-placed Juventus after a 1-1 draw at Bologna, whose injury-time equaliser looked offside.
Samp's city rivals Genoa, two points further back, enlivened their hopes of Europa League football next season with a thrilling 5-3 victory over Cagliari.
Parma beat second bottom Atalanta 1-0 while famous Roman club Lazio remain in deep trouble, two points above the relegation zone, following a harrowing 2-0 home defeat by Bari.
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