Top seed Novak Djokovic was made to work hard by Spain's Roberto Bautista Agut before he posted a 6-3 4-6 6-4 6-3 win at the U.S. Open on Sunday, moving him into his 26th consecutive grand slam quarter-final.
After Djokovic won an opening-set groundstroke battle with steady accuracy and well chosen forays to the net, Bautista Agut used his big forehand to come back from 4-2 down in the second and win the next four games to level the match.
The world number one smashed his racquet after losing the second set but made better use of its replacement, winning the third following a service break in the fifth game, before raising his performances in the fourth set to claim victory.
"Congratulations to Bautista for hanging in there, for fighting. It was a great match tonight," said the Serb, winner of this year's Australian Open and Wimbledon titles.
The Spaniard unleashed 34 winners, including 22 from the forehand, but came up short in a quest to reach his first grand slam quarter-final.
"He made me work for every point," said Djokovic, who will meet Spain's Feliciano Lopez, who beat Italy's Fabio Fognini 6-3 7-6(5) 6-1, in the quarter-finals.
Meanwhile the tennis world No.1 Serena Williams and her sister Venus will square off in the US Open quarter-finals while a concussion's ruled out Canada's Eugenie Bouchard.
Serena Williams served well in moving within three match victories of a rare calendar Grand Slam with a 6-3 6-3 triumph over American compatriot and 19th seed Madison Keys.
Williams made only six unforced errors to 19 for Keys and hit 58 per cent of her first serves.
She took 79 per cent of those points and 70 per cent on her second serves, nearly twice Keys' second-try effectiveness.
"I'm so proud. I'm so glad it's so much better," Serena said of her serve.
"My only chance was to start out fast. I just stayed in there and was able to relax."
Venus Williams, seeded 23rd, downed 152nd-ranked Estonian teen qualifier Anett Kontaveit 6-2 6-1 at Arthur Ashe Stadium, matching her deepest Grand Slam run since reaching the 2010 US Open semi-finals.
"We're both prepared," Venus said. "Even though you're playing your sister you have to be prepared and focus. The preparation doesn't change."
Serena, who holds all four major tennis titles, is trying to complete the first calendar Grand Slam since Steffi Graf in 1988 and match Graf's Open Era record of 22 career Slam singles titles by capturing her Open Era record seventh US Open crown.
But Venus, two years older than Serena at 35, could play the spoiler as she tries to add to a trophy collection that includes the 2000 and 2001 US Opens and five Wimbledon titles, the most recent in 2008.
"I don't think anyone wants to be a spoiler. I think people love to see history being made," Venus said.
"But at the same time, you're focused on winning your match even though the circumstances are really much different than you."
Bouchard, who suffered a head injury after slipping in the locker room on Saturday, pulled out of her match against Italy's Roberta Vinci with what US Open tournament director David Brewer said was a concussion.
Bouchard had pulled out of mixed and women's doubles yesterday.
The 21-year-old from Montreal was a French and Australian Open semi-finalist last year and lost the 2014 Wimbledon final to Petra Kvitova.
Her head injury adds to a season of setbacks, including first-round French Open and Wimbledon exits and a 12-17 record.
Serena leads the Williams sibling rivalry 15-11 but they have split four US Open meetings, including the 2001 and 2002 finals.
Serena won their most recent matchup in the fourth round at Wimbledon in July.
In the men's draw defending champion Croatian Marin Cilic reached the quarter-finals after beating the seeded Frenchman Jeremy Chardy in four sets.
The 9th seed took his record in New York to 11 successive matches after capturing his maiden Grand Slam title in the city last year.
But the 26-year-old had to overcome an injury scare in the second set to stay alive in the tournament when he rolled his right ankle.
Cilic will meet Frenchman, Jo-Wilfried Tsonga in the quarter-finals.