US Open Tennis 2013: Breaking Down Most Dangerous Underdogs on Day 6

There were a couple of big upsets on Day 5 of the 2013 U.S. Open, as Lleyton Hewitt knocked off No. 6 seed Juan Martin del Potro and No. 24 seed Ekaterina Makarova defeated No. 16 seed Sabine Lisicki in straight sets.

Flavia Pennetta also showed against No. 4 seed Sara Errani on Thursday that an upset can happen at any moment, shocking Errani in straight sets.

Pennetta doesn't make my list of most dangerous underdogs on Day 6 (given her 0-5 career record against Svetlana Kuznetsova), but there are several other names to keep an eye on.

Simona Halep (21) vs. Maria Kirilenko (14)

Simona Halep is having a career year on the WTA tour. The 21-year-old Romanian came into the U.S. Open with a 39-14 record and four singles titles this year.

She's beaten such players as Agnieszka Radwanska, Marion Bartoli, Caroline Wozniacki, Petra Kvitova, Samantha Stosur, Roberta Vinci and Kirsten Flipkens. She's also coming off a title in New Haven.

Kirilenko is 31-14 with one singles title this year. She does have a better resume in Grand Slams than Halep—reaching the quarterfinals in the French Open this year and Wimbledon last year—but Halep is on the rise.

Halep has proven she can beat some top players on the women's circuit this year. It wouldn't surprise me if she defeated Kirilenko.

Feliciano Lopez (23) vs. Milos Raonic (10)

Feliciano Lopez is 1-0 lifetime against Milos Raonic, defeating the 22-year-old in Madrid in 2011. He's 22-14 with one singles title this year.

While Raonic owns the better record this year (29-15, one singles title), Lopez has shown he can be dangerous, defeating Kei Nishikori at the Western & Southern Open. 

Lopez also defeated Stanislas Wawrinka this year (although Wawrinka retired in the match), as well as Gilles Simon. 

Karin Knapp vs. Roberta Vinci (10)

Roberta Vinci may be 42-20 with two singles titles this year, but Karin Knapp isn't having a bad year, either.

The world No. 55 has gone 31-18 this year and defeated her first two opponents in the 2013 U.S. Open in straight sets.

Oh yeah, and Knapp owns a 2-1 lifetime mark against Vinci. The fellow Italian defeated Vinci in Rome last year.