Meghan Markle and Prince Harry's new Netflix venture's already poppin' with star power ... 'cause early filming saw Serena Williams hangin' with the duo!
The Sussexes were down in Miami shooting their new show for Netflix all about the polo world ... attending the Royal Salute Polo Challenge Friday.
Check out the pics ... Harry's all decked out in his polo gear -- ready to hop on a horse and score a few points with cameras ready to cover it all.