However, Serena got the better of her counterpart Halep 6-1, 4-6, 6-4 to reach the quarter-finals of the tournament.
Williams, searching for her first Grand Slam title since coming back from pregnancy previous year, has now won nine of her 10 meetings with Halep.
The most telling point of Serena Williams' hard-fought win against world No. 1 Simona Halep didn't happen on match point.
"We have movie night every night but we had it this afternoon because I had a night match tonight, so she loved it", Serena said.
"I really needed to elevate my game, she's the number one player in the world and there's a reason why", she added.
Oil slides on increased US output and US-China trade fears
EST (1727 GMT) versus Friday's settlement price, while USA crude futures CLc1 were up 19 cents to $53.99 a barrel. West Texas Intermediate ( WTI) crude futures CLc1 were up 78 cents, or 1.5 percent, at $52.85 per barrel.
Following Williams' triumphant win, her entire team, including sister Venus and coach Patrick Mouratoglou, burst into wild celebrations. "I have played two really great players over the past few days, and, I don't know, I just have to adjust to that feeling of being overwhelmed a little bit".
"I'm such a fighter, I just never give up.it's definitely something that's innate".
"Doesn't that tell you everything you need to know about Serena Williams?". I'm ready. She's ready.
In that moment, it was tough not to think back to Williams' last match of 2018 in the US Open final, when the pressure of not performing up to expectations and the emotion of what she perceived as an unfair slight by the chair umpire sent her into a tailspin.
Nishikori will now play six-time champion Djokovic, who dropped a set for the second match in a row before seeing off 15th-seeded Daniil Medvedev 6-4, 6-7 (5/7), 6-2, 6-3.
"I have the courage to say that, because I took the risk to stay home so much (for off-season rest)".
Things We Saw On The Streets At The 2019 March For Life
Senate Majority Leader Mitch McConnell (R-Ky.) argued the event was "one of our nation's proudest examples of peaceful activism". The Doug Jones election will be Alabama's own "March for Life".
The 37-year-old American dismantled Halep's game to race through the opening set in 20 minutes.
However, she has tripped up early twice now, dropping the first set to Su-Wei Hsieh in round three and doing the same on Monday against the stubborn Latvian.
At her first opportunity in the next game, she broke the 17th-seeded Keys, the runner-up at the 2017 US Open and a semifinalist or better at three of the past five majors.
And despite strong play on her end, the reigning French Open champion ended up proving herself right, falling to Serena after Williams saved three break points to hold for 3-all in their match's third set.
But as she has time and again done, when the chips were down, Serena just rose to the occasion. "She knew she should not get broken at that time, and she found a way". "'Can you choose another one?'"
Cardi B chastises Trump over shutdown
Cardi B is upset and anxious . "But you concerned about it because you are a citizen of America; you are a citizen of the world". Could this be the start of a political career for Cardi? Bernie Sanders, I-Vt., in advance of the 2016 presidential primaries.