Angelique Kerber's bid for world number one remains on track, as Caroline Wozniacki joined her in the US Open quarter-finals.
Kerber is into the last eight in New York for a second time after overcoming Petra Kvitova in straight sets on Sunday.
A two-time runner-up, Wozniacki made the quarter-finals at a grand slam for the first time since her run to the decider in Flushing Meadows in 2014.
Roberta Vinci and surprise packet Anastasija Sevastova are also into the last eight.
KERBER TOO GOOD FOR KVITOVA, WOZNIACKI WINS
World number two Kerber outplayed an aggressive Kvitova, recording a 6-3 7-5 victory at Arthur Ashe Stadium.
The German Australian Open champion is a step closer to the top ranking - with Serena Williams now needing to reach the final to have any chance of avoiding dropping to second in the world.
-- US Open Tennis (@usopen) September 5, 2016
Wozniacki has endured a difficult year, but she advanced with a surprise 6-3 6-4 win over American eighth seed Madison Keys.
Like Wozniacki, Vinci is a former runner-up - reaching the final last year - and she continued on another run by beating Lesia Tsurenko 7-6 (7-5) 6-2.
Vinci and Kerber will meet in the last eight, while Wozniacki takes on Sevastova after the Latvian upset 13th seed Johanna Konta 6-4 7-5.
Sevastova retired from the sport in 2013 before returning at the start of 2015, with her last-eight berth her best grand-slam run yet.
SERENA UNDER PRESSURE
With Kerber in the quarter-finals, Williams needs to make the decider to have a chance of keeping the number one ranking.
WOZNIACKI UNSURPRISED, SEVASTOVA'S LONG ROAD BACK
Wozniacki may have struggled this year, but she always believed she could go deep at the US Open. "I'm not surprised that I could make it this far because I always believe in myself and I always think that -- you know, when I got on court I always believe that I'll win the match," the Dane said. "But it's nice. It's a nice feeling."
Sevastova spoke about the difficulties after coming out of retirement at the start of last year. "It was a bumpy road," she said. "I didn't expect to be here at this point of my life after retiring, so I don't know what to say really."
WILLIAMS SISTERS FACE TESTS
Serena and Venus are on Arthur Ashe Stadium during the day session. Serena takes on Yaroslava Shvedova, while Venus meets Czech 10th seed Karolina Pliskova. Simona Halep and Carla Suarez Navarro also do battle, while Agnieszka Radwanska faces Croatian Ana Konjuh.