US Open Tennis 2015 Results: Women's Semifinal Scores and Twitter Reaction

The unthinkable happened Friday in the women's portion of the 2015 U.S. Open semifinals.

After inclement weather delayed the women's semifinals from Thursday night until Friday morning, two scintillating matches awaited Arthur Ashe Stadium to decide who would advance to the championship. But instead of the top seeds cruising to easy victories, each found it beyond difficult to navigate any further.

Here's a look at how Friday's women's action from Flushing Meadows shook out.

U.S. Open Women's Semifinals

Match Result	Score
Roberta Vinci def. No. 1 Serena Williams	2-6, 6-4, 6-4
No. 26 Flavia Pennetta def. No. 2 Simona Halep	6-1, 6-3

Roberta Vinci def. No. 1 Serena Williams

Julio Cortez/Associated Press

Arguably the biggest upset of the entire year in sports took place Friday, as Serena Williams' hopes for a calendar Grand Slam were thwarted by unseeded challenger Roberta Vinci. 

To say this one was hard to see coming would be the understatement of all understatements. Pegged as a 1-25 favorite by Odds Shark heading into the match, Williams lost her first Grand Slam match in well over a year.

Williams was just three sets away from tennis immortality and one set shy of victory when she jumped out in front early against Vinci. After allowing an early break to go down 2-1, she battled back to take the opening set. 

But along the way, Vinci survived a Williams surge late in the second set and began to wear down the 21-time Grand Slam champion. Meanwhile, the Italian looked more and more energetic by the point up until her shocking victory that left Arthur Ashe Stadium in disbelief.

Perhaps the only moment better than Vinci's 2-6, 6-4, 6-4 victory was her post-match interview with ESPN's Tom Rinaldi:

It looked apparent that Williams wasn't at her best as the match got into crunch time, but even her best points were matched by superior play from Vinci. For Williams, she couldn't do much but give her opponent plenty of props, as the U.S. Open noted:

Along with the ascension of Jordan Spieth and the U.S. women's soccer World Cup title, Williams looked intent on giving the U.S. sporting faithful another prideful moment by winning all four Grand Slams in one year. That dream is now over, much to the chagrin of the millions cheering her on, as author Ben Shapiro observed:

While it was tough for Williams to walk off the court and head home Friday knowing her hopes for a calendar Grand Slam are over, there's plenty to rest her head on. Namely, a 33-match Grand Slam win streak that reasserted her place—as if there was any question—as perhaps the great women's tennis player ever.

But Friday, none of that mattered. All that seemed to matter was that Vinci was the better player, and that will set up an all-Italian final.

No. 26 Flavia Pennetta def. No. 2 Simona Halep

Robert Deutsch-USA TODAY Sports

It took until the semifinal round this time, but second-seeded Simona Halep suffered a rather surprising defeat for the fourth and final time this Grand Slam season as she fell in straight sets to Italian veteran Flavia Pennetta.

The No. 26 seed won comprehensively, winning 6-1, 6-3 and not allowing Halep much of a chance as she dominated almost every facet of the match. It only took one ace, but Pennetta made her opponent uncomfortable by winning nine of 14 net points and hitting on 69 percent of her first serves.

With the surprising seed making it to the final round, she completed a feat not done since Williams four years ago, as Sports Illustrated's Richard Deitsch noted:

It's not just Pennetta's high seed that has her inching her way into the history books. At 33 years old, the Italian is the oldest woman to make it this far in the entire Open era, as shared by ESPN Stats and Info:

The 23-year-old Halep's struggles were apparent from the get-go, as she couldn't win but 32 percent of her receiving points. With Pennetta easily dominating on her serve, Halep couldn't find the same sort of success with her own service.

After a quick opening set, Halep got fired up after an early break gave her a second-set lead. But just as quickly as she got the lead, she lost it as Pennetta took a stranglehold of the match.

She made the quarterfinals in two Grand Slams this year, but it was otherwise a surprisingly poor season from Halep. She shared her frustration with WTA Insider:

While much of the attention will be focused on her shortcomings, the New York Times' Ben Rothenberg noted Pennetta should get the brunt of the praise:

There's no doubt Halep will be frustrated for the rest of 2015 after a titleless Grand Slam season, but she can take solace in getting all the way to the final four in the year's final major. If she can keep that rolling into 2016, she's guarantee to be Williams' biggest threat in the new year.

As for Pennetta, she's one victory away from turning this already historic ride into a legendary one.