Roger Federer came from behind again to reach round three ©Getty Images

Tennis heavyweights Roger Federer and Serena Williams both had to come from behind to win their round two clashes on a rain-hit day at the US Open.

Defending champion and men's top seed Novak Djokovic and women's second seed Ashleigh Barty also progressed on day three, but showers postponed all action on the outside courts which do not have roofs. 

Switzerland's 20-time Grand Slam champion Federer is looking for a record sixth open-era title at Flushing Meadows in New York but did things the hard way for the second consecutive round.

After dropping the opening set in his first-round win over Indian qualifier Sumit Nagal, the 38-year-old third seed did the same again before beating Bosnian Damir Džumhur 3-6, 6-2, 6-3, 6-4.

He went 4-0 down in the opening set to the world number 99 after hitting 12 unforced errors.

"I clearly have to play better from the get-go," the Swiss said.

"I'm not concerned. 

"It's all good, you know, and I just need to take the positives out of it, because once I lose that first set I do get better, which is a good thing."

Williams, the eighth seed hoping to win a seventh US Open and a 24th Grand Slam on home soil, fell behind in an all-American duel with 17-year-old wildcard Caty McNally.

The 37-year-old, last year's runner-up, bounced back to win 5-7, 6-3, 6-1.

Wet weather postponed all action on the outside courts ©Getty Images
Wet weather postponed all action on the outside courts ©Getty Images

"She came out and played really well and showed no fear," said Williams, the eighth seed.

"She had absolutely nothing to lose and she played like that."

Serena's sister, Venus, was not so lucky as the two-time winner was beaten 6-4, 6-4 by Ukrainian fifth seed Elina Svitolina.

Serbia's 16-time Grand Slam champion Djokovic looked comfortable as he accounted for Argentina's Juan Ignacio Londero, 6-4, 7-6, 6-1.

Australian Barty, the French Open champion, ended the hopes of American Lauren Davis with a 6-2, 7-6 win.

Japan's 2014 finalist and seventh seed Kei Nishikori got the better of home player Bradley Klahn in four sets, coming through 6-2, 4-6, 6-3, 7-5.

There was also success for women's third seed Karolína Plíšková as the Czech player was too strong for Georgia's qualifier Mariam Bolkvadze, 6-1, 6-4.

American Madison Keys, the 10th seed and 2017 runner-up, knocked out China's Zhu Lin 6-4, 6-1.

German qualifier Dominik Köpfer also made the third round as he beat American Reilly Opelka 6-4, 6-4, 7-6.

Action at the hard court event, the last grand slam of the year, continues tomorrow and will include today's matches postponed by the rain.