Novak Djokovic sent a warning to his US Open title rivals with a performance he rated as one of the best of his career.
The world number one dropped only four games in his third-round win over Joao Sousa and was even more emphatic this time, winning the final 13 games of the match to beat Marcel Granollers 6-3 6-0 6-0 in just 79 minutes.
Spaniard Granollers had been taken to five sets in the first three rounds so it was perhaps not surprising he could not give Djokovic a better match.
The Serb has been working on his net play and that was one of several notable statistics, Djokovic winning 28 of 30 points at the net.
His serve is also in good shape and he won the first 25 points on his own delivery, losing just eight in the match.
Djokovic said: "I had some great matches in the past that I can compare with this one, but definitely the second and third sets were some of the best tennis that I've played on Arthur Ashe in my career.
"It all comes at a great time for me. It was something I was wishing, to be more aggressive as the tournament progresses and to be able to stay committed to play every point, to win every point, regardless of the score.
"So I'm very happy with my mental state, how I feel physically, and the way I played. It's definitely coming at the best possible time."
It could not have been more different to Monday evening, when Roger Federer crashed out of the tournament against Tommy Robredo and Rafael Nadal needed more than three hours to see off Philipp Kohlschreiber.
Federer was a shadow of his former self, but Djokovic has urged the critics to lay off.
The Serb said: "People need to give him a break a little bit because I think it's normal to expect that he's not moving as well as he did when he was number one and he was so dominant.
"For me, he's still playing really well and he definitely, with his tennis, deserves to be one of the top five players in the world, no question about it."
Djokovic's next opponent, Russian Mikhail Youzhny, may also be running low on energy after a thrilling five-set win over Lleyton Hewitt.
The Australian, the champion in New York 12 years ago, had rolled back the years with a stunning win over Juan Martin del Potro in the second round and looked set to reach his first grand slam quarter-final for four years.
Hewitt led 4-1 in the fourth set only for Youzhny to win six straight games and lead by a break in the decider.
Back came 32-year-old Hewitt to lead 5-2 but he played a poor game serving for the match and Youzhny reeled off five straight games to clinch a 6-3 3-6 6-7 (3/7) 6-4 7-5 victory after three hours and 58 minutes.
Youzhny is only a year younger than Hewitt but the two veterans covered every inch of Louis Armstrong court, with the crowd being treated to some stunning rallies.
The Russian is a consistent performer at grand slams and has twice reached the semi-finals at the US Open - in 2006, when he beat Rafael Nadal, and 2010.
Hewitt was philosophical in defeat, saying: "Obviously it could have gone either way. There were a lot of momentum changes right from the start.
"It was hard for both of us to hold our serves at times. In the end, he played the big games when he needed to. He didn't give me too many cheap errors.
"It's obviously disappointing to lose but I left it all out there. There's not a whole heap more I could have done."
Andy Murray battled his way past Denis Istomin and into the quarter-finals of the US Open.
It was not a totally convincing performance from the defending champion, who again made a slow start, but he did what he had to do to come through 6-7 (5/7) 6-1 6-4 6-4.
Murray has become a master at negotiating such hurdles at grand slams but he will need to play a lot better if he is to get past old rival Stanislas Wawrinka in the last eight.