Last-child style over-rides first-child style when only one child is available

ul li:first-child a {
   border-radius: 5px 5px 0 0;
}

ul li:last-child a {
   border-radius: 0 0 5px 5px;
}

When there's just once child, the last-child style over-rides the first-child style. Is there a way to apply both (since it is both the first and last child)

Am looking to achieve this with just CSS, without the aid of JS. Thanks.