Sisqó 's offering Cynthia Erivo a front-row seat to the panty party ... offering her a VIP experience at his upcoming London Show after she praised "Thong Song" in an interview.

Here's the deal ... during a chat with Entertainment Tonight earlier this year, Cynthia gave her thoughts on Sisqó's hit track, lauding the guy for pulling out all the stops with his vocals.

Erivo highlights the key change near the end of the track, the impressive runs he goes on, and more ... while laughing about how ridiculous it is that something this impressive is all about an undergarment.

Sisqó tells us Cynthia and her "Wicked" costar Ariana Grande are two of his favorite vocalists -- though he mentions they have a very different style of singing from his -- so, he wouldn't think Cynthia would di

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