Johanna Konta became the first British woman since 1992 to reach the French Open third round as she overcame sickness to battle past Lauren Davis.
Konta, seeded 26th, needed four match points to clinch a hard-fought 6-3 1-6 6-3 win over the American in Paris.
"I've been a bit sick since Sunday. I'm doing everything I can to recover and dealing with it," Konta said.
The 28-year-old British number one will play Slovakia's Viktoria Kuzmova, ranked 46th, on Friday.
Konta would have expected to face fourth seed Kiki Bertens, who Konta beat in the Rome Masters semi-finals recently, but the Dutch player retired from her match against Kuzmova because of illness.
Konta has also been struggling with a heavy cold, blowing her nose between points and sounding hoarse during the post-match media interviews.
"I wouldn't say I'm feeling brilliant, but I'm also managing well and cracking on," she said.
"I'm letting my body heal itself as best as it can with the demands that are on it right now."
- Bertens pulls out as Pliskova and Stephens go through
- Federer & Nadal win in straight sets
- French Open live scores, results & order of play
'I knew it wasn't over until I was shaking hands'
Konta has surged back up the world rankings after a productive clay-court season which has seen her reach the Morocco Open and Italian Open finals, beating Sloane Stephens, Venus Williams and Bertens in Rome.
She had never won a main-draw match in Paris until her first-round win over German qualifier Antonia Lottner on Tuesday.
Now she has a great opportunity to reach the Roland Garros second week after eventually seeing off Davis, who is ranked 111th in the world.
After edging a tight opening set by breaking in the penultimate game, then losing her way in the second, Konta showed impressive mental resilience to eventually take the decider.
She managed to instantly block out the second set, hitting 10 winners and making only two unforced errors to surge into a 5-0 lead.
Then she stuttered again with one game separating her from victory.
Davis saved the first match point on her own serve with a crunching cross-court forehand, then nicked a break back after Konta let slip a 40-15 lead at 5-2.
A hold to love for Davis reinforced momentum had swung back in her favour, with Konta's plight looking precarious when she trailed 0-40 as Davis threatened to put the set back on serve.
Konta, however, managed to reset again, fighting back to avert danger and clinching victory at the fourth attempt with a thumping drive volley.
"I knew up until the very end, until I was shaking hands with her, it was not going to be over," added Konta, who has emulated Briton Jo Durie's achievement of reaching the last 32.
"I'm definitely playing good tennis right now. I'm competing well.
"When I'm presented with tough situations out there, I keep a cool head. I'm managing to problem-solve well, which makes me difficult to play."
'She is tough mentally' - analysis
BBC tennis correspondent Russell Fuller
Only twice in 13 matches this year has Johanna Konta failed to win a deciding set: that is a record which stands comparison with the very best in the world.
Lauren Davis was far too good in the second set, but Konta reasserted her authority from the very first point of the decider. And she then slammed the door shot as a resurgent American threatened to force it open again.
Konta will now play Viktoria Kuzmova for a place in the last 16. The Slovak has a ranking similar to the one Konta had before she reached the final in Rome, but it will be the 21 year old's first ever Grand Slam third round match.
Great Britain Davis Cup captain Leon Smith on Radio 5 Live Sports Extra
I am thrilled for Konta. It is tough to get over the line when sets get tight like that. It is wonderful to see. The third round is such good progress for her.
With Kiki Bertens having to retire from her match, it has opened up the draw. It gives Konta a really good opportunity to get to the second week.
We have seen upsets but she is in a good way. She is tough mentally and has shown good composure.
Evans and Norrie win in men's doubles
Britons Dan Evans and Cameron Norrie fought back from a set down to win 4-6 6-4 6-3 against Americans Steve Johnson and Denis Kudla in the first round of the men's doubles.
Evans and Norrie face the 14th-seeded duo of Dutchman Robin Haase and Denmark's Frederik Nielsen in the second round on Thursday.
In the first round of the mixed doubles, Britain's Luke Bambridge, partnered with American Asia Muhammad, overcame the French pair of Pauline Parmentier and Fabrice Martin, winning 6-7 (5-7) 6-3 10-5.
Briton Neal Skupski and Spain's Maria Jose Martinez Sanchez won via a champions tie-break, beating Japan's Eri Hozumi and Ben McLachlan 6-1 3-6 10-6.
Britain's Dominic Inglot and Romania's Mihaela Buzarnescu lost 1-6 7-5 14-12 to Croatia's Darija Jurak and American Austin Krajicek.