A number of games are down for decision.
Japan's Naomi Osaka has won her first Grand Slam match since pulling out of the French Open.
She defeated Czech player Marie Bouzkova 6-4, 6-1 in the first round of the US Open in New York.
Later on today, top seed Ashleigh Barty will meet Vera Zvonareva.
Andy Murray has been knocked out of the men's singles.
He went down in five sets to Stefanos Tsitsipas having been 2 sets to 1 ahead.
World number one Novak Djokovic is to take on Holger Rune later today.