US Open 2019: All the major results from Day 7

Six-time US Open champion Serena romped to a 6-3, 6-4 victory over Croatian 22nd seed Petra Martic.

Serena battled past an injury scare to set up a quarter-final with China's Wang Qiang, who beat world number two Ashleigh Barty.

Serena ahd required a medical timeout in the second set.

"I feel fine right now physically," said Williams.

She had a heavily tapped ankle.

Chinese 27-year-old Wang extended her best Slam run with a 6-2, 6-4 defeat of Barty.

Barty was undone by 39 unforced errors at Louis Armstrong Stadium.

"It's a tough day at the office today. But it has been a year where we've hit our goals," Barty said.

Meanwhile, Wang said, "I think I'm really focused on the court, really enjoyed the way I played."

Ukrainian fifth seed Elina Svitolina progressed with a 7-5, 6-4 victory over 2017 runner-up Madison Keys.

British 16th seed Johanna Konta overcame Pliskova 6-7 (1/7), 6-3, 7-5.

This was her second win against the third seed in eight tries.

"To go one step further is a massive achievement for me and I'm really hoping to make that one, two, three steps forward," Konta said.

2016 champion Wawrinka led Djokovic 6-4, 7-5, 2-1 when the Serb quit.

He had received treatment on his troublesome left shoulder before the start of the third set, however, he couldn't continue.

"It's never the way you want to finish a match. I'm sorry for Novak. He's an amazing champion," Wawrinka said.

He will meet fifth seed Daniil Medvedev next.

Swiss maestro Federer advanced to the quarter-finals for the 13th time in his past 15 attempts.

He beat Belgian 15th seed David Goffin 6-2, 6-2, 6-0.

"It was great. I thought I had a good feeling out here," Federer said. "David didn't have his best day either, he was struggling."

He will face Grigor Dimitrov next, who beat Alex De Minaur 7-5, 6-3, 6-4.