The American is back on track after a post-US Open slump and she has notched three wins on the trot for the first time since her memorable fortnight in New York last September.
It’s a 10th career victory over a Top 10 opponent for Stephens – and she will challenge Angelique Kerber in the last eight. The German overcame a test from Yafan Wang 6-7(1) 7-6(5) 6-3.
“Garbine is an incredible player, so I knew I had to come out and stay focused on my game,” said Stephens. “I did that pretty well and I just stayed steady the whole time.
“After winning a Grand Slam it’s not easy. I think the long lay-off that I had with injury was a tough time for me. After the US Open I lived life to the fullest and it’s been a little bit difficult. It takes a little bit of confidence and hard work [to get back to winning ways].”
Muguruza impressed in her third round demolition of Christina Mchale, but she struggled to find her groove in the Florida heat this afternoon. She leathered 12 winners, however it was the 32 unforced-errors that was the primary factor behind her sixth loss of a disappointing season.
The Wimbledon champion held serve just once in the opener, and Stephens’ more assured and steady baseline play helped her take it by six games to three.
The second set was a much tighter affair, however it was the American who made a key breakthrough in the seventh game. A sloppy passage of play from the Spaniard – filled with mistakes – enabled the home-favourite to establish a break advantage.
Muguruza tried to muster a response in an effort to prolong the contest, but a lack of consistency meant a comeback was not forthcoming.
Johanna Konta's defence of her title has come to an end. The British No.1 lost in three-sets to Venus Williams 5-7 6-1 6-2. Victoria Azarenka, Elina Svitolina, Karolina Pliskova, Danielle Collins and Jelena Ostapenko complete a strong last eight line-up.