Alex Rodriguez Plays Nice with GF's Billionaire Ex

This COULDA been super awkward ... but Alex Rodriguez apparently played it cool when his girlfriend reconnected with her billionaire ex-husband Sunday night ... at an event PACKED with powerful nerds.

Of course, A-Rod is dating Anne Wojcicki -- who was married to Google co-founder Sergey Brin until they split in 2015.

But it seems they're on good (or at least decent) terms because they didn't just hang out at the event last night ... they teamed up onstage and addressed the crowd at one point during the awards ceremony.

FYI, Brin is reportedly worth more than $30 BILLION.

A-Rod and Wojcicki started dating earlier this year. Brin attended the event with his girlfriend, Nicole Shanahan ... an accomplished lawyer/business mogul (who also happens to be smoking hot).

The event itself was wall-to-wall rich people and celebs ... including Mark Zuckerberg, Vin Diesel, Kevin Durant and more.