The world number one ranking remains firmly up for grabs after Angelique Kerber advanced to the WTA Western and Southern Open quarter-finals.
Ranked second on the WTA Tour, victory in Cincinnati would see Kerber leapfrog world number one Serena Williams.
Australian Open champion and Olympic Games silver medallist Kerber kept her top-ranking bid on track with a 7-6 (7-5) 6-4 win over Barbora Strycova on Thursday.
Romanian third seed and last year's runner-up Simona Halep also progressed to the last eighth, as well as Garbine Muguruza, Svetlana Kuznetsova, Carla Suarez Navarro and Karolina Pliskova.
However, eighth seed Dominika Cibulkova crashed out after losing to Timea Babos.
KERBER EYEING SERENA, HALEP AND MUGURUZA WIN THROUGH
Fresh from winner silver for Germany at Rio 2016, Kerber has hit the ground running in Cincinnati.
She was pushed by Strycova in the opening set but outlasted her opponent in a tie-break thanks to a trademark passing backhand.
Strycova was not overawed in the second set, though she eventually came unstuck in the penultimate game as Kerber broke before serving out the match.
Kerber is now two wins away from climbing to the top of the women's summit.
Halep wasted little time booking her spot in a third consecutive Western and Southern Open quarter-final, dispatching Australian qualifier Daria Gavrilova in less than an hour.
Following another weather-affected day, Halep eased past Gavrilova 6-1 6-2 in just 53 minutes.
As for French Open champion and fourth seed Muguruza, she accounted for fellow seed Anastasia Pavlyuchenkova 7-5 6-1.
CIBULKOVA STUNNED IN LAST 16
The eighth-seeded Slovak huffed and puffed, but fell short against qualifier Babos.
After taking out the opening set, Cibulkova struggled as she lost 4-6 6-3 6-1.
Cibulkova converted her pair of break points in the first set, but the 2014 Australian Open runner-up failed to make the most of her opportunities thereafter, unable to covert the next six break points on offer.
HALEP EXTENDS WINNING STREAK, PLISKOVA PUTS ON SERVING MASTERCLASS
After making light work of Gavrilova, Halep saw her winning run extend to 12 matches. She tallied 18 winners and 14 unforced errors in the routine victory.
Suarez Navarro's defense was on show during her 6-1 7-5 triumph over fellow seed Roberta Vinci, with the Spaniard fending off seven of 10 break points.
Czech 15th seed Pliskova fired down 15 aces as she accounted for Misaki Doi 7-5 6-3, while seventh seed Kuznetsova prevailed 6-2 3-6 6-2 against 12th seed Timea Bacsinszky after more than two hours on court.
KERBER ENJOYING 'INCREDIBLE' YEAR
"I've had a lot of ups and downs in my career, but I'm having the best year of my career and it's still not over," said the 28-year-old. "It's amazing what's happened in the last few months - it's just incredible!"
STARS RETURN FOR QUARTERS
Kerber headlines Friday's schedule as she faces Suarez Navarro on Centre Court. Halep is set to meet Radwanska/Konta. Pliskova and Kuznetsova will go head-to-head, while Babos stands in the way of Muguruza and a semi-final berth.