Federer breezed past Krajinovic, courtesy of his dominant serve. Williams dropped just five games in her win over Zidansek.
While Federer and Williams advanced on Wednesday, there were plenty of upsets elsewhere, with several men's seeds bowing out of the tournament:
Novak Djokovic, Naomi Osaka, Coco Gauff and Ashleigh Barty all won their matches Wednesday.
Selected Men's Results
(3) Roger Federer bt. Filip Krajinovic: 6-1, 6-4, 6-1
(32) Milos Raonic bt. Christian Garin: 6-3, 6-4, 6-2
Tommy Paul bt. (18) Grigor Dimitrov: 6-4, 7-6 (6), 3-6, 6-7 (3), 7-6 (3)
(6) Stefanos Tsitsipas bt. Philipp Kohlschreiber: Walkover
Tennys Sandgren bt. (8) Matteo Berrettini: 7-6 (7), 6-4, 4-6, 2-6, 7-5
(2) Novak Djokovic bt. Tatsuma Ito: 6-1, 6-4, 6-2
(9) Roberto Bautista Agut bt. (WC) Michael Mmoh: 5-7, 6-2, 6-4, 6-1
Selected Women's Results
(8) Serena Williams bt. Tamara Zidansek: 6-2, 6-3
Coco Gauff bt. Sorana Cirstea: 4-6, 6-3, 7-5
Caroline Wozniacki bt. (23) Dayana Yastremska: 7-5, 7-5
(3) Naomi Osaka bt. Saisai Zheng: 6-2, 6-4
(7) Petra Kvitova bt. Paula Badosa: 7-5, 7-5
(1) Ashleigh Barty bt. Polona Hercog: 6-1, 6-4
(10) Madison Keys bt. Arantxa Rus: 7-6 (3), 6-2
For the full results and stats, visit the Australian Open's official website.
Federer came out firing, needing just 20 minutes to take the opening set. Krajinovic looked completely overwhelmed, as he had no answer for the Swiss star's serve.
He played with more aggression in the second set and found a little more success, although the 38-year-old continued to produce magical shots like this:
An early break in the third set broke Krajinovic's resolve, and Federer comfortably played out the rest of the match.
Williams remains perfect on the year after her win over Zidansek, even though she didn't hit her best form on Wednesday. The 38-year-old made a few too any mistakes from the base line but was able to keep her nerve at key times.
She was in good spirits after the match:
The top-seeded women's players took care of business on Wednesday. Hercog never came close to pulling off the upset against Barty after a dreadful start, and Osaka overcame a game Zheng to set up a highly anticipated clash with Gauff.
Zheng made the defending champion work for her win:
Gauff dropped the first set against Cirstea and came within two points of a loss, but the 15-year-old rising star showed incredible resilience to battle back and take the win.
Defending men's champion Djokovic enjoyed a comfortable outing against Ito, a wild card who has never made it out of the second round at a Grand Slam. While he had some struggles with the wind in the second set, the Serb was never truly troubled by the 31-year-old from Japan.
He took to Twitter after the match:
Both Berrettini and Dimitrov suffered shock exits in five-set losses against Sandgren and Paul, respectively. The Bulgarian in particular wasted opportunities and had the win slip through his hands:
Both Dimitrov and Berrettini were in Federer's quarter of the draw, joining the likes of Denis Shapovalov and Borna Coric on their way out and leaving Fabio Fognini and Guido Pella as the only seeded players remaining.