Serena Williams faces an Australian Open litmus test after shifting into survival mode to make it through the first week at Melbourne Park for a mind-boggling 16th time.
The seven-times champion overcame a shaky start to beat Russian teenager Anastasia Potapova 7-6 (7-5) 6-2 on Friday.
Up against a player half her age, 39-year-old Williams had to recover from 5-3 down and save two set points in the first set before prevailing over the former Wimbledon junior champion.
Potapova served for the opening set at 5-3 only to cough up two double faults to invite Williams back in.
The 19-year-old then led 5-3 in the tiebreaker before losing four points in a row to concede the set and match momentum.
Potapova took an injury time-out while trailing 4-1 in the second set, but it couldn't stop Williams from advancing after an hour and 37 minutes.
"It definitely feels good to be in the fourth round and it's so good to get through that match," the 10th seed said.
"The first set was extremely tight. I was a little tight, but it worked out. I was able to play a little more free in the second set.
"It's about surviving and playing better every round."
Bidding for a record-equalling 24th grand slam singles crown, Williams will certainly need to be better next round.
The American will take on in-form seventh seed Aryna Sabalenka on Sunday for a quarter-final spot.
Sabalenka has never previously made it past the third round at Melbourne Park and only reached the final 16 once at a grand slam tournament - the US Open in 2018.
But the Belarusian came into the tournament in stellar form and has won 18 of her past 19 matches, including Friday's 6-3 6-1 dispatch of American Ann Li.
The big-hitting 22-year-old won the WTA 500 title in Abu Dhabi last month, her third straight title, and described her first match up against Williams as a "thrill".
"It's a big challenge, but it's a tough one, but I'm going to do everything I can to prepare myself for that match," Sabalenka said.
"She's huge and she's a great player, and it's always I think a big challenge to play against these kind of players."
Williams knows she could be in for a torrid time, too.
"Well, she hits very hard. She has a big, big power game. She's a big girl. Strong like myself," she said.
"So I think it will be a really good match - with no crowd."
