When asked about her close friendship with the Duchess, Serena gushed over Meghan, saying she is "literally the strongest person" she knows and the "nicest".
She continued: "She flew all the way with a newborn to see me play in New York and flew all the way back that night and I probably couldn't have done that.
"It kinda goes to show you what an amazing person she is and that's just one of the many things that she does for me."
Serena went on to say that despite the fact Meghan is now a member of the British royal family, she still makes time for her – no matter how far apart they are.
"I call her, text her anytime, crying, upset - she's always there. Doesn't matter what she's going through, she just sets time away and she's just so amazing," she said.
Serena’s candid confession about her BFF comes after it was revealed last week that Meghan also makes time for her other pals, including talk show host Ellen DeGeneres.
During a recent show, Ellen revealed to viewers that she invited a pair of dancers onto her chat show after Meghan texted her about how 'great' they were.
The talk show host delighted the performing duo when she told them how she came across their video.
"Meghan Markle sent me this video, she texted me and said what do you think of this, how great is this girl?" she said.