US Open 2013 Schedule: Breaking Down Most Dangerous Underdogs on Day 5

Flavia Pennetta's upset of No. 4 seed Sara Errani on Day 4 of the 2013 U.S. Open once again proved that anything is possible in a Grand Slam.

The world No. 83 came into this year's tournament with a 16-17 record, but managed to knock off one of the top women's players in the world (and a fellow Italian, at that). Not only that, but Errani was also the defending U.S. Open champion.

So, which underdogs do we need to watch out for on Day 5?

Here are my top picks headed into Friday.

*Complete schedule of play can be found at USOpen.org

Laura Robson (30) vs. Li Na (5)

Laura Robson was only 13-17 this year coming into the U.S. Open, but she is 1-0 lifetime against Li Na and she defeated her first two opponents in New York in straight sets.

Moreover, Robson's win against Li came at the U.S. Open last year. She defeated Li in three sets, 6-4, 6-7, 6-2. 

The world No. 32 has had a rough year by her standards. She was a combined 79-55 in her past three years before 2013. The law of averages suggests she will get things going in the later part of this year.

It's also worth noting that Robson is, 19 years old, so the Brit has plenty of room to grow. 

Jamie Hampton (23) vs. Sloane Stephens (15)

Jamie Hampton is 1-2 lifetime against Sloane Stephens. She defeated the 20-year-old in straight sets in Charleston last year.

Stephens is widely considered to be a rising star, but it's also worth nothing that she came into the 2013 U.S. Open with a 29-18 record, while Hampton came into the major with a 28-18 record. 

And while Stephens has defeated Hampton this year, as well as Serena Williams and Maria Sharapova, Hampton has also posted some impressive wins, including victories over Agnieszka Radwanska, Caroline Wozniacki, Petra Kvitova, Roberta Vinci and Kirsten Flipkens.

Lleyton Hewitt vs. Juan Martin del Potro (6)

If you simply looked at the rankings and/or seedings of Lleyton Hewitt and Juan Martin del Potro this year, Hewitt would appear to have no chance on Friday.

Then again, while Hewitt isn't the same player who won the 2001 U.S. Open, he can still be dangerous. 

Case in point: Hewitt is 2-2 lifetime against Del Potro. That includes beating Del Potro in the pair's latest matchup in London this year. 

Hewitt also played John Isner tough in Atlanta in late July after beating him in Newport. That was after upsetting world No. 10 Stanislas Wawrinka at Wimbledon.

It's also worth noting that Del Potro is coming off a match that lasted four hours and 13 minutes. The final three sets each lasted 70 minutes or more.