Top matches of 2015: 9) Roberta Vinci over Serena Williams

Serena Williams reacts after losing a point to Roberta Vinci. Photo / AP

US Open semifinal - Roberta Vinci over Serena Williams, 2-6, 6-4, 6-4

Serena Williams won an amazing 26 straight grand slam singles matches this year but it's the one defeat that will stand out for a long time.

After victories at the Australian Open (her sixth career title in Melbourne), the French Open (her third) and Wimbledon (her sixth), Williams was odds-on favourite to capture her 22nd career grand slam title and the elusive Grand Slam with victory at the US Open.

She looked like an even more dead cert after dropping just two sets on her way to the semifinals where she came up against unseeded Italian Roberta Vinci. Ranked 43rd in the world, Vinci had never won a grand slam semifinal and in their four previous match-ups, the Italian hadn't even taken a set.

Vinci's unusual style, full of slices and net rushes, kept Williams off-balance enough to cause problems and prevent the 33-year-old American from becoming the first player since Steffi Graf in 1988 to win all four Grand Slam tournaments in a calendar year.

How Serena Williams lost to unseeded Roberta Vinci

Simply put - Vinci had one of her best days on court while Serena had one of her worst. Williams was undone by 40 unforced errors, twice as many as Vinci.

The 2-6, 6-4, 6-4 defeat ended one of the great runs in the history of tennis. Not only was Williams 26-0 at grand slam tournaments this year, but her winning streak at majors was 33 matches, because she won last year's U.S. Open.

If she had managed to win a fifth consecutive major title, Williams would have raised her total to 22, equaling Graf for the most in the Open era, which began in 1968, and second-most in history behind Margaret Court's 24.

Instead she was stuck with a second 'Serena Slam'.

Vinci went onto lose to compatriot and childhood rival Flavia Pennetta in the final. Williams took the rest of the season off to get over an injury and the defeat.

- With AP