By Chris Oddo

(March 25, 2013) -- Serena Williams looked like she needed a shot of espresso during the first set of her round of 16 encounter with the always feisty Dominika Cibulkova in Miami today.

Men's Action: Murray and Gasquet Move into Miami Fourth Round

Perhaps it was the early afternoon start, her sister Venus's recent health woes, or some unresolved personal matter that the 15-time Grand Slam champion couldn't shake free of.

Whatever it was, it would not last any longer than the typical espresso shot does in the hands of a caffeine junky, as Williams flicked the switch and kicked her game into rarefied gear to deny Cibulkova the upset, 2-6, 6-4, 6-2 in just under two hours.

Read: Approaching 32, Serena Williams Paying Attention to Her Legacy

"Mentally I was just fried and I was just thinking about everything but tennis,” five-time Sony Open champion Williams told reporters after the match. “Then I just had to pull myself back together.”

Moments after Williams had pulled herself together, America's second-highest ranked woman was coming apart at the seams, as Sloane Stephens surrendered all the momentum and an early lead in a disheartening 4-6, 6-2, 6-0 loss to Agnieszka Radwanska.

Williams reversed her fortunes with a grounded grit and determination, ripping nineteen winners while erasing a 4-1 deficit in the second set versus Cibulkova, while Stephens' fortune seemed to float away just like the sometimes mercurial mind of the 20-year-old who has become America's darling thanks to a buoyant personality and a run of eye-opening results in 2013.

Read: Stephens Celebrates 20th Birthday with Ellen This Week

The matches said so much about the two American's involved; Williams, who could seemingly dig herself out of a Grand Canyon-sized deficit once she sets her mind to it, remains at the pinnacle of the sport because of her mental strength. Stephens, who has the moxie and the athleticism to reach the pinnacle someday (perhaps sooner than later), is perhaps still a year or two away from maximizing her full potential, because of her lack of it.

But don't misunderstand. Stephens does have a good mental game. It's just a work in progress, while Williams' is a work of art.

“You can always take something from it and get better,” Stephens presciently said to reporters after her loss. “That's always a good thing.”

Stephens was serving to level the second set, down 3-2, when she surrendered the break that would tilt the match on its ear. “I mean, I definitely felt I should have held serve at 3-2,” she said afterwards. “It didn't happen, and it is what it is.”

From there Radwanska ran roughshod over her, winning nine consecutive games to close out the match, including seventeen straight points at one juncture.

“Well, you know, she's very good, young player and always difficult to play with,” Radwanska said. “I remember our match from last year. I won 6-4 in the third, so I knew it wouldn't be easy today.”

It was certainly easier in the third set, and Stephens will have to live with the reality that she disappeared from a match that she was very close to winning at some point. It's a bitter pill to swallow, but one that will likely make Stephens even stronger down the road.

Also See: Tweets of the Week: Serena Rides a Bike, Maria's Sarcasm

As for Williams, she couldn't have been any stronger than she was down the stretch against Cibulkova. During a tense third set, Williams clung to a slim lead, using her cool, calm presence and big, blistering serve to keep the gap.

Known for her power and aggressiveness, the subtle nuance of Williams' shot selection and her ability to angle the ball both short and wide were both on full display. Even with a lead, she kept up the pressure, sensing the kill, and not wanting to let the momentum swing back in her opponent's favor. She rallied back from 15-40 to take a double break lead at 5-2, then served two aces in the final game, including her 13th of the match on match point.

“This particular time I just thought to myself, Well, I've been down worse,” said Williams. “It's nothing new. Just keep fighting. I never give up. Doesn't matter whether it's in life or on the tennis court, I keep fighting. That's what I kept doing today.”

Li, Vinci Reach Quarters

Fifth-seeded Li Na took out Spanish wildcard Garbine Muguruza, 7-6(6), 6-2, to set up what is sure to be a highly anticipated quarterfinal clash with Serena Williams.

Roberta Vinci eased past Alize Cornet 2-6, 6-4, 6-4. The 15th-seeded Italian will face 30th-seeded Kirsten Flipkens in the quarterfinals. Flipkens needed three sets to defeat Croatian qualifier Ajla Tomljanovic, 6-7(6), 6-4, 6-3.

(Photo Credit: AP)