 1, Serena Williams, albeit the American retired due to a knee injury.She was beaten by Karolína Plíšková and Elina Svitolina in the respective ties against the Czech Republic and Ukraine.At the Australian Open, Wolfe was forced to retire, a set and 2–4 down in the second, in her first-round match against Anastasija Sevastova after sustaining a back injury during the warm-up.