Contrary to OP's thread title, there's no indication she can't "handle the pressure." She says she want's to pursue other dreams and spend time with her family. Seems like a sign of maturity and good mental health to me. Being a top-tier tennis player on the pro tour sounds glam, but for a lot of people it's a grind.
Also, she's retiring from playing professional tennis, not retiring altogether. She probably has other interests and life goals, including starting a family. My hunch is that maybe one of her parents or grandparents is dealing with a serious health issue and she wants to spend time with them too.
What looks like a dream job from the outside isn't always that great; and not everyone finds life in the public eye and on the road to be fulfilling, even when it's lucrative. Over the years, there have been several notable cases of musicians who played with top rock bands finding out that they didn't care for the life once the bands started to garner success. Graham Simpson left Roxy Music of his own volition after cutting the band's first album, which was a huge hit in the UK and Europe. The original guitarist with the Stooges decided he'd rather go back to college after Iggy and the band first made it big; he went into tech and became big in the microchip industry. One of the original members of the Clash left to pursue other things; I think he became a chiropractor. Doesn't mean any of 'em couldn't handle the pressure.