Katie Boulter and Harriet Dart both won at the Nottingham Open to set up an all-British quarter-final.
British number one Boulter, 26, overcame Ukrainian lucky loser Daria Snigur 7-5 6-3 in a tricky encounter at the grass-court event.
Dart, ranked 143rd in the world, then beat Ukraine's Anhelina Kalinina 6-0 7-5 to reach the last eight.
Andy Murray is in last-16 action against France's Hugo Grenier later on Thursday.
Boulter holds the top British women's ranking for the first time in her career after reaching the semi-finals of the Surbiton Trophy last week and jumping ahead of Emma Raducanu, who has been hampered by injuries.
Against Snigur, Boulter squandered seven break points in the sixth game of the opening set with the Ukrainian eventually prevailing to take a 4-2 lead.
But despite her mistakes, persistence paid off as Boulter broke back to level 4-4 and went on to take the first set in one hour and 15 minutes.
Snigur knocked out the Brazilian second seed and defending champion Beatriz Haddad Maia in the first round but seemed to be hindered by a knee problem against Boulter.
The Briton went down another break early in the second set, but Snigur's knee issues meant she eventually needed a medical time out midway through the second set.
Boulter capitalised on her opponent being evidently uncomfortable, winning three games in a row before confidently seeing the match out and reaching a third quarter-final on the Nottingham grass.
"It was an absolute battle, I'm really proud of myself," said Boulter. "I put my heart on the line and played as well as I could.
"A couple of those games were ridiculous. It was so tough."
Harriet Dart has reached the quarter-finals at Nottingham for the second consecutive yearDart set up a quarter-final against her compatriot with two wildly contrasting sets against fifth seed Kalinina.
The 26-year-old Briton dominated the first set to win it without dropping a single game as Kalinina struggled to get going.
But, after Dart received treatment to what seemed like a lower-back strain, she was made to work for the second, going a break down but battling back to secure a late break of her own and progress.
"I hope to play more sets like that [first set]," said Dart. "I knew I had to come out firing - I'm just really pleased to be able to come through in two sets."We're [Boulter] good friends but let the better player win tomorrow!"
Elsewhere, Briton Jodie Burrage faces Poland's third seed Magda Linette in her last-16 match.
In the men's doubles, British duo Liam Broady and Jonny O'Mara secured a comfortable 6-1 6-4 win over India's Yuki Bhambri and Saketh Myneni to reach the semi-finals.
Joining them in the last four are fellow Britons Jacob Fearnley and Johannus Monday, who beat Anirudh Chandrasekar and Vijay Sundar Prashanth 7-6 (7-4) 6-3, and the British pairs could meet in the title match should they both win their semi-finals.
