World number one Victoria Azarenka scraped into the quarter-finals of the Sony Ericsson Open, surviving a two hour 49 minute battle with Slovak Dominika Cibulkova 1-6 7-6 7-5.
The victory was Azarenka's 26th straight, but came after she was dominated in the first set.
Cibulkova, playing with bags of energy and invention, was well placed at 4-1 up in the second but lost out in a tie-break.
The Slovak was within two points of victory on five occasions in the tightly fought third set but each time Belarusian Azarenka fought back.
Azarenka will now face France's Marion Bartoli in the quarter-finals.
Serena Williams produced 20 aces and blasted her way into the last eight with a 7-5 6-3 win over Australian Samantha Stosur.
It was not all plain sailing for Williams, who lost her first service game of the match but came back to grind out the set in the first meeting between the two players since Stosur beat the American in the 2011 U.S. Open final.
Down 5-2 in the second, Stosur twice saved match point and forced Williams to serve out for the match.
"I think for the most part things are working pretty decent," said Williams, who said revenge for her defeat at Flushing Meadows had not entered her head.
Williams will face Caroline Wozniacki on Tuesday after the Dane overcame Yanina Wickmayer.
Russian Maria Sharapova also sealed a place in the quarters with a 6-4 7-6 win over compatriot Ekaterina Makarova despite being broken four times.
Makarova fought particularly hard in the second set but eventually fell 7-3 in the tiebreak.
"She had opportunities and served for the second set but I just hung in there and stayed consistent. I played a much smarter tiebreak," she said.
Sharapova was hampered by a worrying 11 double-faults but said she wasn't overly concerned.
"It's not easy to play with that but I managed to win the game in two sets, so that's a plus," she said.
Sharapova, the runner-up here last year, will face China's Li Na after she beat German Sabine Lisicki 3-6 6-4 6-2.
Later on Monday, Venus Williams faces Ana Ivanovic.
