Maybe, just maybe, Novak Djokovic will work up a sweat in the U.S. Open's fourth round.

He certainly has not been forced to expend much energy so far.

After playing a total of 31 minutes after the first round, the No. 1 seed and defending champion plays for a berth in the quarterfinals Sunday night in Arthur Ashe Stadium against 84th-ranked Kyle Edmund of Britain.

Djokovic did not play a point Wednesday, because his second-round opponent, Jiri Vesely, withdrew a couple of hours before the match, citing an arm injury. And then Djokovic only contested six games Friday, until his third-round opponent, Mikhail Youzhny, stopped because of a strained left hamstring, prompting the 12-time major champion to head back out on court with coach Boris Becker for a practice session.

So the question was put to Djokovic: Is so little action a good thing, because it provides a chance to rest and heal after some trouble with his left wrist and right arm recently? Or is it a bad thing, because there's a chance of accumulating some rust?

"Well, depends at how you look at it," he began.

"This particular situation I never had in my Grand Slam career. But considering the stage of the season, you know, the amount of matches I've played, what I've been through with my body, I think it's actually good to have some days off and then shorter matches — from one side," Djokovic said.

"From the other side, sure, as you are approaching (the) second week of the Grand Slams," he continued, "you want to have match play and you want to have time spent on the center court before you face one of the top players."

Edmund and Andy Murray gave Britain two men in the round of 16 at the American Grand Slam tournament for the first time in 50 years, back when it was known as the U.S. Championships. Edmund is the first man from the country other than Murray to get this far at any Grand Slam tournament since Tim Henman was a semifinalist at the 2004 U.S. Open.

Then there's this daunting stat for Edmund, who eliminated 20th-seeded John Isner of the U.S. in the third round: Djokovic has a 72-match Grand Slam winning streak against opponents ranked outside the top 50.

Here's what else to watch Sunday at the U.S. Open:

KEYS VS. WOZNIACKI: There will be a matchup of vastly contrasting styles when No. 8 Madison Keys takes on two-time runner-up Caroline Wozniacki in the afternoon on Ashe. Keys, a 21-year-old American, has among the most powerful serves and forehands in women's tennis today. Her aggressive approach will go up against the more conservative, defensive game of Wozniacki, a former No. 1 who is now ranked 74th. Wozniacki hadn't won at a match at a major tournament in 2016 until this week.

NADAL VS. POUILLE: Rafael Nadal has been playing about as well as can be so far in New York heading into his match against No. 24 Lucas Pouille of France, dropping only 20 games — his lowest total through three matches at the U.S. Open, which he won in 2010 and 2013. The left wrist injury that sidelined him for 2 1/2 months does not appear to be much of an issue any more for the 14-time major champion. Pouille is coming off his Grand Slam quarterfinal debut at Wimbledon.

TSONGA VS. SOCK: Two guys equipped with big serves and big forehands meet when No. 9 Jo-Wilfried Tsonga of France plays No. 26 Jack Sock, the last U.S. man in the tournament. Tsonga was the runner-up at the 2008 Australian Open, while Sock will be trying to get to his first major quarterfinal — and trying to be the first American man to make it that far at the U.S. Open since Andy Roddick and John Isner in 2011.

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