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Inter Milan's hopes of qualifying for the Champions League are hanging in the balance after they slumped to an embarrassing 4-1 loss to Napoli.
Napoli fullback Mario Rui struck two minutes from time to give his side a dramatic, rain-soaked 2-1 win at SPAL on Sunday.
Lorenzo Insigne's 98th-minute penalty helped Napoli clinch the runners-up spot in Serie A with a 2-1 home win over Cagliari.
Napoli midfielder Adam Ounas suffered a cut knee after an attempted mugging following his side's 2-0 victory away at Frosinone.
Napoli pushed Frosinone towards relegation with a 2-0 away victory as Dries Mertens equalled Diego Maradona's league scoring record.
Atalanta surged into contention for a top-four finish in Serie A with a 2-1 victory at Napoli on Monday.
Arsenal beat Napoli to book their second straight Europa League semifinals appearance, and will now take on Valencia for spot in the final.
Napoli postponed Juventus's march to the Serie A title and consigned Chievo to relegation with a 3-1 win at the Stadio Bentegodi on Sunday.
Arsenal have launched an investigation into a video which appears to show a supporter racially abusing Napoli defender Kalidou Koulibaly.
Serie A future and past helped Arsenal see off Italian opponents, but this Europa League quarterfinal's open first leg suggests the tie is not over.
Arsenal beat Napoli 2-0 in the first leg of their Europa League quarterfinal tie on Thursday.