US Open burning questions: Is Djokovic the favorite? Can Stephens repeat?

Six different players encompassing both the ATP and WTA have walked away with the first six Grand Slam titles of the year. Despite the hegemony of the Big Four, no male player has successfully defended the US Open title in a decade, while two of the past three women to win Slams, Caroline Wozniacki and Simona Halep, were first-time major champions.

As usual, there are lots of burning questions on the eve of the last Grand Slam tournament of the year. Here are five of them, with ESPN tennis analyst Patrick McEnroe weighing in:

Given the result in Cincinnati last week, is it Novak Djokovic versus the field once again?

No doubt about it, Djokovic's win over Roger Federer -- which also completed his full set of Masters 1000 titles -- was massive. The 31-year-old reigning Wimbledon champion, seeded No. 6, has burst out of his slump in a big way. After a trickier-than-it-looks first-round match against rapidly developing Hungarian Marton Fucsovics, Djokovic may have clear sailing to the fourth-round match potentially against No. 17 seed Lucas Pouille. But a rematch with No. 2 seed Federer looms in the quarterfinals, unless explosive floater Benoit Paire or No. 30 seed Nick Kyrgios derails the Swiss icon. Djokovic or Federer could then find himself facing No. 7 and 2014 US Open champion Marin Cilic for a place in the final.

McEnroe's take: "I still put [No. 1 seed] Rafael Nadal as the tournament favorite, although it's very slight. He and Djokovic had that spectacular match at Wimbledon. Nadal was a swing or two from winning despite the fact that Djokovic's game is much better suited to grass. Djokovic turned a big corner in Cincy, but he had very tough matches. He still seems a little more vulnerable than Rafa, who's had a lot of wins this year -- including a big one on the hard courts in Toronto just weeks ago.

In the grand scheme, is it really important for Serena Williams to do well at this tournament?

Yes and no. Yes in that she was a Wimbledon finalist, but she has just a 1-2 record in hard-court matches since then. Not to mention a strong run would send a message to her rivals moving forward. No, in that the No. 17 seed has nothing to prove to anybody with her amazing record (23 Grand Slam singles titles, six in New York). Besides, Williams still believes her comeback is just beginning. This year would then seem like a prelude to a major, perhaps final, push in 2019.

The draw gods have not been kind to Williams. She could meet sister Venus in the third round and top seed Halep in the fourth. If Serena survives to the quarters, she could meet No. 12 Garbine Muguruza with defending champion Sloane Stephens a potential semifinal opponent.

McEnroe's take: "Serena is an awesome example, not just to mothers but people in general. It's admirable that she's even trying to get back into the workplace and having this platform is a big part of what's driving her. But the reality is that she wants to win. The hard courts have been more problematic for her than the grass at Wimbledon. At the Open, she will have to hit more balls and rely a little less on her serve. But she's still the ultimate competitor and nobody should ever forget that."

Who are the most likely players to surprise us by going deep in the tournament?

No. 4 seed Alexander Zverev has had loads of trouble cracking the Grand Slam code despite his high ranking and proficiency at Masters 1000 and lesser events. He may finally punch through to become a headliner in Gotham. He starts with a qualifier (one of four in his quarter), and his biggest stumbling block before the quarterfinals looks to be the slumping and oft-injured No. 21 Kei Nishikori or No. 13 Diego Schwartzman. This is a prime opportunity for Zverev.

Among the women, the choices are manifold because of the depth on the WTA Tour. Kiki Bertens, No. 13, is 2-1 in WTA finals this year, with a huge win last week in the Cincinnati final over US Open top seed Halep. Bertens has a clear path to a winnable fourth-round meeting with No. 2 seed Wozniacki.

McEnroe's take: "Let's go with real surprises: Stefanos Tsitsipas on the men's side, Amanda Anisimova among the women. Tsitsipas is a huge talent, different from Alexander Zverev or Dominic Thiem in that he's really multidimensional. He can do everything, but it may take him a little longer to put it together. Anisimova is just 16 and the junior US Open defending champion. She's got everything and a lot of people think she's ready to prove it. The women's field is wide open; you can make a case for a dozen women to win it."

What will it take for Sloane Stephens to defend her championship?

Given recent US Open history, Stephens has her work cut out for her if she hopes to defend. On the other hand, she seems impervious to expectations -- good or ill. Stephens is 6-2 in main tour finals but never has defended one. She has room to operate in Flushing this year, starting with a first-round match against Evgeniya Rodina, followed by a qualifier. Wild card Victoria Azarenka is a potential third-round opponent, but Stephens has won their previous two meetings this year. Stephens' only two losses in finals have been to Halep (French Open and Montreal, both this year). Halep looms as a potential semifinal rival.

McEnroe's take: "At her best, Stephens has the offense, defense, and consistency to dominate anyone on hard courts. Halep toughed her out a few times this year, but at the Open, Stephens' A-game will be hard to deal with. She may not have that fear factor that Serena inspires, but there's a great likelihood that if Sloane doesn't go out in the early rounds, you'll probably see her in the late stages of the tournament."

Will we see a Nextgen breakthrough on the men's side?

The younger generation seems to be taking turns stoking expectations, but that's what "development" is all about. At the moment, Tsitsipas is hot, while Denis Shapovalov is treading water. Hyeon Chung, the Australian Open semifinalist, is still fighting back from injury but rounding into form. Frances Tiafoe is making steady progress and certain to have the audience backing him in New York. It's like water building up behind a dam, and all it will take for the dam to break is for some of the over-30 veterans to falter. But we've been saying that for some time now.

McEnroe's take: "Alexander Zverev hasn't broken through at a Grand Slam yet. His quarterfinal at the French Open was his best major thus far. Would I be shocked if the Nextgen players produced two quarterfinalists? No. But at the same time I believe that with the physicality and quality of the older guys, we're more likely to see a Marin Cilic or Juan Martin del Potro pull off a big surprise against the top three."