Swiatek beat Ons Jabeur in last year's finalVenue: Flushing Meadows, New York Dates: 28 August-10 SeptemberCoverage: Daily live text and radio commentaries across the BBC Sport website, app, BBC Radio 5 Live and 5 Sports ExtraIga Swiatek took just 49 minutes to beat "best friend" Kaja Juvan and reach the US Open fourth round.
The pair have known each other since they were teenagers but defending champion Swiatek was ruthless in a 6-0 6-1 victory in New York.
Top-ranked Swiatek made only one unforced error in the first set and dropped just 15 points in the match.
"It is literally like playing against your sister because we've known each other for so long," the Pole said.
"I didn't like the fact that I was winning against my best friend, but I knew that I have to be really focused and not let myself think about that.
"I don't have many friends but she's my best friend so that was the toughest part for sure."
After winning the first set in 24 minutes, Swiatek raced out to a 3-0 lead in the second.
When Slovenian Juvan finally won the next game to ensure she avoided a dreaded 'double bagel' she waved her arms and looked up to the sky in relief, with the crowd loudly cheering her on.
Swiatek, however, remained completely focused, winning the next three games to clinch the match before the pair shared a warm embrace at the net.
They won gold playing together at the Youth Olympics in 2018 and have previously spoken about sharing a love of readingexternal-link.
Swiatek will play Jelena Ostapenko, who she has not beaten in three attempts, or American Bernarda Pera in round four.
