Victoria Azarenka overcame an injury-hampered Jamie Hampton 6-4 4-6 6-2 in a thrilling third round clash at the Australian Open in Melbourne.
The world number one outlasted her American rival, who suffered a lower back injury in the second set when the contest was firmly in the balance, after two hours and nine minutes on court.
There was little indication of what was to come in the early stages though, as Azarenka surged into a 5-1 lead in the first set.
But then came the first sign of wobbles from the defending champion as she served a double fault on set point - one of three in a sloppy game in which she was eventually broken.
And after holding, Hampton broke again as Azarenka's mental breakdown continued. But, as so often happens in tennis, having done the hard work when the pressure was off the world No.63 promptly crumbled when serving at 4-5.
Azarenka did little to bring up two more set points and then profited when the American coughed up a double of her own.
To her credit, Hampton responded well at the start of the second. She established a 3-1 lead in the and maintained the advantage before bringing up two set points on the Azarenka serve at 5-3.
While nerves again got to her as she made two basic mistakes, allowing Azarenka to escape immediate danger, of bigger concern was the back problem she sustained while hitting a forehand.
She returned bravely from a medical time-out to serve out the set but the problem would hamper her movement in the decisive third set. She was in obvious pain when hitting on her forehand side, although somehow still managed to break in the third game to edge ahead.
Yet with cramp also causing her discomfort, Hampton lost five games in succession to lose to Azarenka, who was full of praise for her beaten opponent after the match.
"She played incredibly. She went for every single shot," she said. "All credit to her, she has a great future in front of her, she has great potential.
"She took a medical time-out but she was ripping winners all over. I thought 'can I have a back problem?!' I was feeling great but missing every shot."
Azarenka will face Elena Vesnina in the next round after she eliminated 16th seed Roberta Vinci with a 4-6 7-6 (7-4) 6-4 win.