Roberta Vinci stunned world number one Serena Williams at the 2015 US Open, winning 2-6, 6-4, 6-4 in two hours and two minutes. With this win, Vinci ends Williams' bid for a Calendar Grand Slam and makes her maiden Grand Slam final in singles competition. In the final on Saturday afternoon, she will play her compatriot Flavia Pennetta who upset number two seed Simona Halep in straight sets.

Dominant First Set from Williams

In the opening set, both players looked to have settled in nicely and it was Vinci who was able to get the first look at break point opportunities in the third game, thanks to some aggressive play. It was clear that the Italian knew that hitting backhand slices and not going for her forehands were not going to win her the match, as she learned from her meeting with the world number one in Toronto, just a month ago. At 1-1 with Williams serving, both women had their chances but it was Vinci who converted her second break point, courtesy of a forehand unforced error from the American.

Now up 2-1 and a break, the Italian number three was looking to consolidate the break, which is easier said than done. Williams was not going to let her do that as some fierce hitting got her the break right back. The world number one was taken a little by surprise, by how aggressive Vinci was playing. As a result, she had to up her level considerably. From there, Williams showed why she is arguably the best female tennis player of all time, with some incredible serving and groundstrokes.

The 21-time Grand Slam champion held to love to go up 3-2, and broke once more to go up 4-2. With another great service game, 4-2 quickly became 5-2 for Serena as she was another game closer to the rare Calendar Grand Slam. Despite some good resistance from Vinci, it was a case of third time lucky for Williams as she won the first set 6-2, much to the delight of the crowd.

Vinci Hangs on to Crucial Break to Win the Second Set

In the opening game of the second set, Vinci raced out to a 0-40 lead on Williams' serve but the world number one hit back, winning five points in a row with some stunning serving and passing shots. From there, both women continued to hold serve for the next three games before another break point opportunity presented itself. At 2-all with Williams serving, Vinci found herself up 40-0 on Serena's serve. Could she convert one of these three opportunities? The first two were saved but third time was the charm as the Italian broke to go up 3-2. But could she hold on to that advantage?

In the end, that one break of serve proved to be enough for Vinci and Williams were both able to hold all of their respective service games for the rest of the set. The world number one had a total of two chances to get the break back but she had a couple of chances to give herself more opportunities than that.

The problem for Serena was that she made unforced errors when those chances/half-chances presented themselves. However, take nothing away from Roberta Vinci. She played superb tennis to keep her nose in front. Vinci's slice backhand and aggressive forehand proved to be a deadly combination that would work well against any player, let alone the world number one. That combination seemed to work well as Roberta's consistency was enough to ware down a nervous-looking Serena, as the Italian won it by six games to four.

Vinci Holds Her Nerve, Completes Upset

To begin the final set, Williams began in great fashion. The world number one was waiting for that one opening in rallies to go for her shots. This tactic seemed to work well as Serena held and broke to go up a 2-0 lead. Now with her back up against the wall again, Vinci was desperate to make something happen so she wouldn't fall further behind. She didn't need to change much as her consistency and variety continued to give Williams trouble, and it was enough to get the all-important break straight back. Now trailing 1-2, to say Vinci needed to hold serve was an understatement, but the Italian held her nerve and serve to level the second set at two-games-all. The next two games went to the server and all of a sudden, both women were just three games away from the U.S. Open final. Williams looked to be on her way to another routine service hold but Vinci refused to let up, hitting this winner on the world number one's first game point.

That incredible point from both women proved to be one of the turning points in this match, as Roberta would go on to break for a 4-3 lead. In the next game, Williams began to play more aggressive and even if Vinci was defending as much as she could, Serena gave herself two break point opportunities to break right back. But once again, unforced errors began to creep into the world number one's game, and the Italian number three would go on to hold serve. Now trailing 3-5, Williams still had the belief that she could turn this match around and it was evident as she held to 15, forcing Vinci to serve for the match.

As you would expect, both women were very nervous, for different reasons. However, it was the Italian who was being the more aggressive and tactically smarter player of the two. As a result, she was rewarded with three match points. Vinci only needed the one to complete the upset! Roberta Vinci had defeated world number one Serena Williams 2-6, 6-4, 6-4 in a couple of minutes past the two hour mark. Roberta couldn't believe it! With her left hand clasped over her face, the 32-year-old had done the unthinkable. Not only did she make her maiden Grand Slam singles final, she had denied Williams her bid for history.