Heading into the US Open, Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray have won 36 of the past 38 Grand Slam titles, a stretch dating to the 2005 French Open.

Nowadays, there seems to be a growing sense?- or hope, maybe?- among the best of the rest on the men's tennis tour that the quartet might be more vulnerable than ever.

Nadal, the 2013 champion at Flushing Meadows, is sidelined by an injured right wrist. Federer is 33 and hasn't won a major championship in more than two years. The No. 1-ranked Djokovic is coming off a triumph at Wimbledon, yes, but he also has a .500 record since. Murray had back surgery and hasn't reached a final anywhere since winning Wimbledon more than a year ago.

Others took note when Stan Wawrinka won the Australian Open in January (Juan Martin del Potro's victory at the 2009 US Open makes him the only other interloper in this era).

"Maybe the gap has closed a little bit. That's in large part due to other players believing. That's the most important thing when you're up against a guy like Rafa, Roger or Novak. You just have to believe that you can beat those guys,'' said John Isner, the only seeded American man in New York at No. 13.

Here are other things to watch at the US Open, the hard-court tournament in New York that begins Monday and ends Sept. 8:

NEW KIDS: If there is going to be a first-time men's major champion, two popular picks are Milos Raonic of Canada and Grigor Dimitrov of Bulgaria. Each is 23; each made his Slam semifinal debut at Wimbledon. While Dimitrov has never won a US Open match, Raonic reached the fourth round the past two years. All six of Raonic's titles came on hard courts, and some opponents consider his serve the sport's best. So does he. "I have a lot of belief in my serve,'' Raonic said. "I know how hard it is to break me and how well I can take care of my serve against anybody.''

SERENA'S MAJOR CONCERN: Serena Williams could become the first woman in nearly 40 years to win three consecutive US Opens, but she has not been past the fourth round at a major in 2014. After a third-round loss in singles at Wimbledon, Williams made a bizarre exit from doubles, whiffing on practice strokes, having trouble grabbing tennis balls, and quitting after four double-faults in a row.

HARD TIME ON HARD COURTS: Can't list contenders without naming Maria Sharapova, whose five major championships include the 2006 US Open. Or Eugenie Bouchard, the only woman to reach three Grand Slam semifinals in 2014. Neither, though, has looked great on hard courts lately: Since Wimbledon, Sharapova is 4-2, and was pushed to three sets in three of those wins; Bouchard is 1-3.

INJURIES TAKE A TOLL: In addition to Nadal, who is skipping the US Open for the second time in three years, China's Li Na?- the No. 3-ranked woman and one of this season's Grand Slam champions?- is absent with a knee injury. Del Potro has a wrist injury. Tommy Haas is out after shoulder surgery. There are plenty of players in the draw with various problems, including two-time runner-up Victoria Azarenka, who missed time with a foot injury and more recently had a bad right knee.

ROOF ON THE WAY: After all the rain in recent years, the first significant steps have been taken toward putting a retractable roof on Arthur Ashe Stadium. The project is expected to cost more than $100 million and be ready for the 2016 US Open. It got underway in earnest over the past six months, when steel rods were driven 125 to 180 feet into the ground outside the octagon-shaped arena. What's visible now are concrete pedestals, some covered with plants and one housing a bar, for this year's tournament.