Serena Williams reveals why Meghan Markle is her best friend

Turns out, Serena Williams and Meghan Markle are both the best kind of friends to have!

The 37-year-old tennis pro has been longtime pals with the Duchess of Sussex, Meghan Markle. During Markle's Royal Tour of Australia, New Zealand, Fiji, and Tonga in October, she wore a blazer from Williams' clothing collection, which took Williams by surprise.

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"We were all just coming out of a photoshoot for the brand when we heard that Meghan wore our line," she told Marie Claire. "We had sent stuff to her a while ago, so we were excited to see it. I've known her for years, and it's cool to see your friend help you out without even trying."

The Duchess had just announced her first pregnancy days before rocking the look, and all eyes were on her. It was a key time to through her friend a fashion credit.

"She just wanted to look good and wear a blazer. The next thing you know it’s our blazer," Williams said. "I was like, 'Meghan, thanks!' And she was like, 'Why are you thanking me? We're friends!' "

Looks like Williams takes that friendship very seriously as her response to the Duchess rocking her duds was to create even more pieces in her honor.

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"Since that blazer sold out, now we just have special pieces that are specifically made for Meghan and we send those to her," she revealed. "It's been fun creating them."

Williams and her husband, Alexis Ohanian, attended Markle's May Royal Wedding to Prince Harry at Windsor Castle.

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The friends have remained close since Markle became a member of the royal family. Back in September, Williams told Australian TV's "The Sunday Project," "We were actually just texting each other this morning … We have known each other for a long time, but we really kind of are relying on each other a lot recently."