The top seeds made it look easy on a hot and humid Labor Day.
Djokovic, the 2011 champion at Flushing Meadows, made only 19 unforced errors, saved both break points he faced, and breezed past 22nd-seeded Philipp Kohlschreiber of Germany 6-1, 7-5, 6-4 Monday.
Bellis played on the grandstand court, Flushing Meadows’ third-largest venue, to accommodate her following after her performance in the main draw.
The second-seeded Federer was a game from losing the first set to Marcel Granollers in the third round Sunday when play was stopped because of lightning.
The sisters are bidding for their 14th Grand Slam doubles title, but first at Flushing Meadows since 2009.
10th seed Caroline Wozniacki dazzled on the Ashe Stadium Court in the third set, beating 5th seed and 2014 French Open champ Maria Sharapova 6-4, 2-6, 6-2 to advance to the quarterfinals.
Wozniacki beat the five-time major champ 6-4, 2-6, 6-2 in 2 hours, 37 minutes on a steamy Sunday.
The No. 1-ranked and No. 1-seeded Williams has faced American opponents in all three of her matches this week while extending her overall winning streak at Flushing Meadows to 17.
If the highest-ranked American man wanted to, he’d have plenty of material in order to prepare for the task he faces Saturday: facing 22nd-seeded Philipp Kohlschreiber of Germany in the third round of the U.S. Open.
Second-seeded Roger Federer of Switzerland has not won a grand slam in the US Open in more than two years, and at 33, some have wondered whether his time might be up.
Two points from defeat, Sara Errani rallied to force a third-set tiebreaker and outlasted Venus Williams in a back-and-forth match at the U.S. Open.
While the defending men’s champion, Rafael Nadal, withdrew due to injury before the tournament started, there are still enough stars on both the men’s and women’s side to keep demand high.
CiCi Bellis started nervously, then won seven games in a row during one stretch, but in the end, was beaten 6-3, 0-6, 6-2 by 48th-ranked Zarina Diyas of Kazakhstan.
Ivanovic, who had been having her best season since winning the French Open in 2008, lost to Karolina Pliskova 7-5, 6-4 in the second round Thursday.
Two-time champion Venus Williams won a place in the third round of the U.S. Open Wednesday night for the first time since 2010, a year before she announced she had been diagnosed with an autoimmune disease.