Experts' picks: Not everyone a believer in Djokovic, Serena

What a whirlwind the past couple of months have been on tour.

Novak Djokovic, once unbeatable, has come earthbound rapidly, while Serena Williams, after a dominant run at Wimbledon, enters the final Grand Slam of the season with confidence question marks and shoulder issues.

What to expect at the US Open? Truth is, we really don't know, but our experts make their best guesses in the final picks of the year.