Serena Williams has won the Australian Open, the French Open and Wimbledon this year. A victory in the U.S. Open would give her the Grand Slam for 2015. (John Minchillo/Associated Press)

The United States Open tennis championship starts Monday, and all eyes will be on Serena Williams. The 33-year-old superstar is trying to complete a Grand Slam.

A tennis player is said to achieve a Grand Slam by winning the four major tournaments — Wimbledon and the Australian, French and U.S. Opens — in the same calendar year.

Williams has won the Australian Open, French Open and Wimbledon this year. So a victory in the U.S. Open in New York would complete her Grand Slam.

Grand Slams in tennis do not happen very often. The last woman to win all four majors in a single year was Steffi Graf in 1988. The last man to complete the Grand Slam was Rod Laver in 1969.

Five years ago, I wrote a column wondering who was the greatest woman tennis player of all time. I picked Steffi Graf over such legends as Margaret Court, Martina Navratilova (pronounced nav-ruh-til-LO-vah) and Chris Evert. I picked Graf because she completed the Grand Slam, won 22 major championships and won each of the four major championships at least four times.

Steffi Graf, shown in 1996, completed a grand slam and won 22 major championships. She retired in 1999. Williams is just one championship behind Graf. (Mark Lennihan/AP)

I did not pick Serena Williams because five years ago she had won “only” 13 major titles. I also figured Williams would not win many more majors because she was getting old — she was 28 at the time — and getting injured more often.

A lot has changed since September 2010. Williams has added eight major championships — one Australian Open, two French Opens, two Wimbledons and three U.S. Opens — to her trophy case.

Now she has 21 major championship wins. That’s more than Navratilova and Evert (who had 18 each) and only one fewer than Steffi Graf. And Williams is still playing.

So I’ve changed my mind about who is the greatest woman player of all time.

Sometimes you might hear on sports radio or in presidential debates that it’s a bad thing to change your mind. People call it “flip-flopping.”

But it’s okay to change your mind if you discover new information or have a good reason to change your mind. It’s what thinking people do. Serena Williams has given me eight very good reasons in the past five years to change my mind.

So watch the U.S. Open and Serena Williams during the next two weeks. I think you will be watching the greatest of all time.

Bowen writes the sports opinion column for KidsPost. He is the author of 21 sports books for kids. His latest soccer book, “Out of Bounds,” was published this month.