Victoria Azarenka extended her 2012 winning streak to 20 matches on Tuesday, but Caroline Wozniacki crashed out of the BNP Paribas Open.
World No 1 Azarenka made short work of Julia Goerges, needing just 63 minutes to post a 6-3 6-1 victory and reach the quarter-finals.
In the quarter-finals she will face Polish fifth seed Agnieszka Radwanska, who led Jamie Hampton 6-3 4-6 3-0 when the American retired hurt.
A recent winner in Dubai, Radwanska is another of the WTA's form players - she has won 20 of her 23 matches this season, only losing to Azarenka - so the Belarusian is expecting a tight battle in the last eight.
Battles
"I think it's going to be a tough match, as usual. We have really tough battles," said Azarenka. "I'm really looking forward to playing her again. She's a great player and a very good challenge for anybody to play."
However, Azarenka's predecessor as world number one, Wozniacki, bowed out to another former top-ranked player, Ana Ivanovic.
Ivanovic stunned Wozniacki by posting a 6-3 6-2 win.
Second seed Maria Sharapova also made light work of her fourth-round match, beating Italy's Roberta Vinci 6-2 6-1, to set up an all-Russian quarter-final against Maria Kirilenko.
Kirilenko beat another Russian, Nadia Petrova, 6-1 5-7 6-2, while there were also wins on Tuesday for Marion Bartoli, French Open champion Li Na and Angelique Kerber.
