After a week of lukewarm performances, four-times champion Federer turned on the heat with a masterful 6-3 6-3 6-2 destruction of Czech 28th seed Radek Stepanek.
As temperatures soared on day seven of the hardcourt major, an ailing Marion Bartoli said it was a "miracle" she could battle on for more than three hours in the stifling conditions. She eventually fell 7-6 0-6 6-4 to Austrian Sybille Bammer.
"I had very low blood pressure and very weak pulse and the trainer advised me not to continue, especially because it was so hot," said the 12th seed, who had her pressure checked on court midway through the first set.
"I was sick since yesterday morning because I caught a stomach virus. Since then I had not eaten anything except one glass of orange juice."
While Bartoli wilted under sunny skies, Federer flourished.
In his first two matches, the Swiss master had fallen well short of proving his title credentials as he piled up 66 unforced errors and converted only 11 of 28 break points.
On Sunday, he dazzled Stepanek and 23,000 wide-eyed fans on Arthur Ashe Stadium court when his game clicked into overdrive.
At one point, the Swiss even managed to surprise himself. With his back pinned to the rear wall, Federer leapt high into the air and his slam-dunk, mis-hit bounced off a startled Stepanek’s racket frame for a winner.
After toying with Stepanek for 96 minutes, Federer threaded a forehand down the line to seal victory and celebrated by punching the air and holding aloft his index finger, as if to remind people "I’m still number one in New York."
"At the end of the day, what counts is winning the tournament. You forget all the unforced errors you made," said Federer, whose cloak of invincibility has been looking rather threadbare recently following his defeats in the French Open and Wimbledon finals by Rafael Nadal.
"If I were to win one of those slams right away, I have the invincibility factor again. I don’t try to impress anybody in the early rounds," said the 27-year-old, who has also surrendered his number one status to Nadal.
Federer chalked up his 30th consecutive victory here to set up meeting with Russia’s Igor Andreev.
Second seed Jankovic was glad to reach the quarter-finals for the third year running after being pushed to the limit in her previous two matches.
The Serbian repeatedly complained to the umpire about being distracted by flying debris on a windy Arthur Ashe court but regained her focus to book a last eight showdown with Bammer.
"I couldn’t really find my rhythm in these kind of conditions," complained the outspoken Serb, who is the highest woman’s seed left in the draw.
With the top ranking up for grabs following the shock defeat of Ana Ivanovic in the second round, Jankovic admitted she wanted to fulfil two objectives by next Saturday.
"It (the number one ranking) is a goal, but I want to win a grand slam," said Jankovic, who has yet to reach a major final.
"Of course, by winning a grand slam I will regain the number one ranking again, and that is something I want really bad. I will try my best to achieve that."
Olympic champion Elena Dementiava continued to slice through the draw with a ruthless 6-4 6-1 win over China’s Li Na. Next up for her is Swiss 15th seed Patty Schnyder.
Qualifier Gilles Muller probably entered the Open with more modest ambitions than Jankovic or Dementieva but he put Luxembourg on the tennis map on Sunday.
The 130th-ranked Muller squeezed past Spanish 18th seed Nicolas Almagro 6-7 3-6 7-6 7-6 7-5 to become the first player from the tiny nation to reach the fourth round of a grand slam.
He will now run into fifth seed Nikolay Davydenko, a 6-2 7-6 6-3 winner in an all-Russian clash against Dmitry Tursunov.
Eighth seed Andy Roddick was likely delighted to have secured an early night in after beating Italian Andreas Seppi 6-2 7-5 7-6. The American’s last match, against rising Latvian Ernests Gulbis, finished at 1.34 am local time. With Reuters