Thuram netted all three of Inter Milan's goals in the defending champions' 3-2 win over 10-man Torino while Retegui helped Atalanta beat his former club Genoa 5-1.
Inter moved to second on the table, two points behind Napoli after Antonio Conte's team beat Como 3-1 on Friday.
Just one point separated Inter and Torino coming into the match but the visitors' chances of getting something at San Siro diminished in the 20th minute when defender Guillermo Maripán was shown a straight red card for a dangerous tackle on Thuram.
Inter led five minutes later when Thuram headed Alessandro Bastoni's cross in off the inside of the left post.
Thuram doubled Inter's tally in the 35th with another towering header, this time from a cross by Francesco Acerbi, but Duván Zapata reduced the deficit almost immediately.
Marcus Thuram celebrates amid his hat-trick for Inter Milan. (AP PHOTO)
Thuram completed his first hat-trick for Inter on the hour mark. Torino goalkeeper Vanja Milinković-Savić could only parry a header from Lautaro Martínez and Thuram managed to bundle home the rebound despite being off balance.
Thuram was given a standing ovation when he was substituted off eight minutes later.
However, there was a nervy finale for Inter after Nikola Vlasic netted a penalty for Torino with three minutes remaining following a foul from Hakan Çalhanoğlu on Adam Masina.
Retegui didn't celebrate his three goals in respect for Genoa.
Genoa were the first Italian team Retegui played for after joining from Argentine outfit Boca Juniors in 2023.
The Argentina-born Italy forward spent a season at Genoa before moving to Atalanta in the offseason.
After each of his goals against his former club, Retegui just held up his hand almost in apology before he was embraced by his Atalanta teammates.
Retegui opened the scoring in the 24th and doubled his tally early in the second half when he fired home a rebound after Genoa goalkeeper Pierluigi Gollini - who is on loan from Atalanta - parried Ederson's effort.
Atalanta's Mateo Retegui was muted in his three goal celebrations against his former club. (AP PHOTO)
Ederson got on the scoresheet 10 minutes later, and Retugui completed his hat-trick by converting a penalty following an Alessandro Vogliacco handball.
Marten de Roon netted a stunning volley before Jeff Ekhator scored a consolation for Genoa.
Jordan Zemura set Udinese on the way to a 1-0 win over Lecce with a magnificent free kick.
Udinese had already hit the woodwork twice in an entertaining match before Zemura curled into the top right corner from 25 metres out in the 25th minute.
Udinese moved into third while Lecce remained fourth from bottom after their third straight defeat without scoring.
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