And in tennis, Serena Williams has separated herself from the rest of the flock. Even though she fell short of pulling off a magical feat last week, her year has still been one of the greatest displays in the history of the sport.

You can make a very good argument that the two most dominant athletes in the world right now are women.

In the world of mixed martial arts, Ronda Rousey has utterly destroyed her competition--allowing her to become the most popular fighter in the world--male or female.

And in tennis, Serena Williams has separated herself from the rest of the flock. Even though she fell short of pulling off a magical feat last week, her year has still been one of the greatest displays in the history of the sport.

She came so close to pulling off something that has only been done once. Williams lost in the semifinals of the U.S. Open--preventing her from winning all four open championships in the same tennis season.

If she would have pulled it off, she would have tied the only other woman to pull off that feat back in 1998, Steffa Graf, for most career open-era Grand Slam titles with 22.

Margaret Court still holds the record for most championships overall with 24.

Despite the hiccup, Williams' dominance over the past year has been quite impressive.

The thing that makes the accomplishment all the more extraordinary is that she came so close to puling it off at the age of 33--which is usually when tennis players are considered over the hill. Graf was just 18 when she did it.

Williams was already thought to be one of the top female tennis players of all time before she started her 2015 championship run.

Now, with her near calender Grand Slam and with her being just one title short of Graf's championship record, many have gone on to coronate her as the greatest player the sport has ever seen.

As great as Williams is and as great as this past year has been, I think many have forgotten just how amazing Graf was back when she was obliterating the competition.

Williams' greatness is unquestioned, and she's certainly one of the best to ever pick up a racket.

She is sure to tie Graf for the most championships in the open era, and she will inevitably pass her up and set a new mark.

However, I still think Graf is the greatest female tennis player of all time.

There is one factor I look at that leads me to that conclusion: competition.

You can make an argument that Williams was just as dominant or even more dominant. One thing you can't argue is the fact that Graf had to deal with much tougher opponents during her career.

During Williams' era of dominance, the next best player was probably her big sister Venus. Venus has won seven career Grand Slams.

But other than Venus, Serena really hasn't had many top-eschalon contenders to deal with.

Maria Sharapova has had a great career, but she'll never be considered one of tennis' all-time great players.

On the other hand, Graf had to deal with some of the sport's greatest juggernaughts.

On that list was Monica Seles--who won nine career Grand Slams, eight before she was 20.

She was stabbed in the back by a fan in 1993. It put her out of action for two years, and she was never the same when she returned.

If not for that horrific incident, who knows how many championships she would have won.

Most notably, Graf had to deal with two top-five players in Martina Navratilova and Chris Evert. Each of those women won 18 championships apiece.

Another amazing thing about Graf--in her amazing run to the calender Grand Slam in '88, she won 54 of 56 sets.

This argument is the same argument I use against Roger Federer in the men's division.

Federer has eclipsed Pete Sampras for most career Grand Slams with 17, and even though I admire Federer's dominance and I consider him one of the all-time greats, I still think Sampras is the man.

Once again, Sampras played much stiffer competition during his career.

He had to deal with guys like Andre Agassi, Jimmy Connors, John McEnroe, Stefan Edberg and Boris Becker. Those guys won 35 Grand Slams titles between them.

During Federer's dominance, there really wasn't a top-tier guy to challenge him.

Andy Roddick only won one career Grand Slam; Andy Murray has won just two.

By the time Rafael Nadal and Novak Djokovic broke through and began reeling off championship victories, Federer's best years were behind him.

Both men have had plenty of success against him in the past five years. Djokovic just edged him out for the U.S. Open title this past weekend.

To be the best, you have to beat the best.

Sampras played in what I believe to be the greatest era in men's tennis, and he beat all of the best players in his run to 14 titles. To me, that makes him the best men's player of all time.

Graf routinely duked it out with and beat the sport's best in Navratilova and Evert to claim 22 championships. As amazing as Serena is, to me, that makes Graf the best women's player of all time.