Williams couldn't have been more dominant in her rout of Carla Suarez Navarro, dominating from start to finish in a 6-0, 6-0 victory.
The other top seed in the tournament, Novak Djokovic, was battling in the first set with Marcel Granollers before rattling off 13 straight games en route to his own near-perfect performance.
More struggles beset No. 3 seed and 2012 U.S. Open winner Andy Murray, but he eventually pulled through against a resilient unseeded opponent in Denis Istomin after dropping the opening set in a tiebreaker.
Below is a look at the complete scores from Day 9, along with a recap of the major developments at the year's final Grand Slam event.
Note: Match statistics and information are courtesy of USOpen.org, unless otherwise indicated.
No. 1s Live Up to Billing
Every facet of the top-ranked players' games seemed to be on, and what stood out for Djokovic was his peerless volleying.
On the occasions when Djokovic crashed forward, he won 28 of 30 net points, which helped him hit 34 winners and just 15 unforced errors. Meanwhile, Granollers had numbers of eight and 20 in those same categories and was simply outmatched after winning his last game at 5-3.
But at least Granollers can say he walked away having won three games, because that's more than Suarez Navarro can lay claim to.
Williams stated after the match on the ESPN telecast that she didn't feel it was her best tennis, even though the scoreboard against her No. 18-seeded counterpart seemed to indicate otherwise.
With only three winners to 12 unforced errors, Suarez Navarro was powerless against the intimidating Williams, who won 88 percent of her first-serve points, converted six of seven break chances and won 12 of 15 net points.
Other Big Names Fight to Advance
Murray capped off the evening at Flushing Meadows by grinding it out against Istomin, whose returning became more of a liability as the match progressed.
It marked the third match in a row that Murray had dropped a set, but he has come back admirably on all three occasions, this time bouncing back by crushing Istomin 6-1 in the second set before fighting for 6-4 wins in the subsequent ones.
Ana Ivanovic beat second-seeded Victoria Azarenka in the first set, but couldn't keep up the momentum in a back-and-forth match where both players struggled to gain traction. Azarenka had a huge advantage at the net and broke her opponent nine times but surrendered seven breaks herself.
Better serving will be necessary for Azarenka to stay on track for the final.
Li Na couldn't close out Ekaterina Makarova in two sets but dominated the last set 6-2 after losing a tight tiebreaker. She now has the unenviable task of facing Williams in the semis, though.
Perhaps the highest quality of tennis overall came from Stanislas Wawrinka, who played magnificently against Tomas Berdych, beating him in a see-saw four-setter.
Both players hit the ball with staggering velocity and displayed outstanding fitness. Just when it looked as though Wawrinka would take control at two sets to one, he double-faulted to allow Berdych back in at 5-5:
However, he bounced back and closed out the third-set tiebreaker with a thunderous ace.
Berdych lost his spirit from there, as Wawrinka won his third consecutive set 6-2 to advance to the quarterfinals. He will have a difficult road ahead in facing Murray, but given the Scot's difficulties lately, if Wawrinka plays as he did on Tuesday, he can't be counted out.
Hewitt's Run Ends
Mikhail Youzhny dropped the second and third sets before battling back to narrowly beat the feisty Aussie in a thrilling match. It wasn't the prettiest display of tennis, as Hewitt committed 54 errors to Youzhny's 63.
Hewitt, the 2001 U.S. Open champion, actually won more total points than Youzhny did, but did make his share of mistakes, double-faulting nine times and generally not looking as sharp as he had in his previous triumphs.
There was no visible doubt that Hewitt left it all on the court as he has throughout his career.
It would have been a whole different task for Hewitt to oust Djokovic in the quarterfinals, but given that he'd taken down Del Potro in Round 2, it could have been feasible.
Instead, Youzhny will give Djokovic his best shot, but will need to recuperate quickly and prepare for the men's top seed to have any hope of matching his previous best in the U.S. Open by reaching the semifinals.