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Caroline Wozniacki is missing her best friend. The retired tennis star opens up about how the pandemic has taken a toll on her longtime friendship with fellow athlete Serena Williams in this

week's issue of PEOPLE. "Serena has been in her house as well and been really, really careful," Wozniacki — who hails from Denmark — says of their separation caused by the

coronavirus crisis. "And I've been on the other side of the world and, and even when I was in America, we were both just in our houses and we were discussing how we could come and

see each other." She continues, "There was one point where we were in St. Louis, where David is from, and that was a 15-hour drive [to Serena in Miami, Florida]. So that was a

little excessive." But there are already plans for the 30-year-old to reunite with Williams, who lost in the U.S. Open women's singles semifinals on Thursday. Says Wozniack,

"I said, you know, when we're back on the same continent, we'll have to get tested and make sure everything's okay so we could see each other." The close friends

have supported each other for numerous life events, with Williams, 38, even serving as bridesmaid in Wozniacki’s Tuscany wedding to former NBA star Lee in June 2019. The pair was supposed to

play each other in an April exhibition match in Copenhagen, which was ultimately canceled due to COVID-19. It would have been Wozniacki's final match after retiring following the 2020

Australian Open. "That was something I really looked forward to, and I had wanted to say a proper goodbye to my Danish supporters and fans who have supported me for so many years,"

she explains to PEOPLE. "That was definitely one thing that I hope will still happen — and I think this will happen in the future — but that was something I didn't get to do at

the planned time." _FOR MORE ON WOZNIACKI AND HER BATTLE WITH RHEUMATOID ARTHRITIS, PICK UP A COPY OF PEOPLE, ON NEWSSTANDS FRIDAY_ Wozniacki's retirement came after she learned

that she had rheumatoid arthritis, an incurable autoimmune disorder that causes stiff and painful joints. She said the choice to walk away from tennis was not because of her RA, though the

disease does “make some things more challenging” — but to spend more time with Lee and start a family.