This essay is one of more than 20 nominations for SI's 2014 Sportsman of the Year. You can see all of this year's nominees here and vote for your choice for SI Sportsman here. This story also appears in the Dec. 8, 2014, issue of Sports Illustrated. Subscribe to the magazine here.

When you go through hard times, you find out who your true friends are, and Serena Williams was one of the first people to be there for me when my engagement unraveled last spring. She didn’t have to support me—she has her own life, her own problems and her own career to worry about—but she was there when I needed someone to lean on the most. We talked on the phone, and she told me that it was going to be hard to recover, but that when I did get to the other side, I was going to be a stronger person. And now I am.

My 2014 Sportsman nominee: Goalkeeper Tim Howard

On the court in 2014, Serena did not have the greatest year by her Hall of Fame standards. There were injury withdrawals, the illness at Wimbledon and early exits at a handful of tournaments. But she also won a singles title at the U.S. Open—the 18th Grand Slam of her career—and six other WTA titles, including the Miami Open, the Western & Southern Open in Cincinnati and the Brisbane International in Australia. It’s just so impressive that she has been so good for as long as she has. This is her 18th year on the Tour, and she’s averaged 3.6 tournament wins each year, for a total of 64. But it’s not just her athletic accomplishments that are admirable. She inspires me as a person as well, and that’s why I’m choosing Serena as my Sportswoman of the Year.

Serena is always concerned about her family and friends. She stays true to her word and never changes her personality for anyone or anything. Throughout her career she has remained consistent as a person, and I think that is really admirable.

Wozniacki celebrates with Serena after finishing the 2014 New York City Marathon in November. Alex Trautwig/Getty Images

I find Serena to be a great role model for young kids, whether they have aspirations to play professional tennis or not. When it came to off-court business interests, she didn’t simply sell her name to a manufacturer, she started her own clothing line—and not only helped design the products but also took a hand in establishing the styling and branding of the operation. Also, it’s so important to engage with fans, and she is always signing autographs and taking pictures with a smile. Her charity fund, The Serena Williams Foundation, has done everything from supporting two schools in Kenya to giving scholarships in the U.S. to providing disaster relief in Haiti. She and her sister Venus have even helped revamp a tennis center in Washington, D.C. Serena’s efforts to give back to others are such a positive representation of tennis as a sport.

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When we’re facing one another on the court, we are battling for the same prize and we do our best against each other because we want to win. Everything else is put aside. At the year-end championships in Singapore in October, she beat me in the semifinals, 7–6 in the third set, and then went on to win the event. It was a tough loss for me, and it still stings, but it wasn’t the only time she got the better of me. I lost to her four times this year: at the Rogers Cup in Montreal, at the Western & Southern, in the final at the U.S. Open and in Singapore. She’s easily been my toughest competitor this season.

When you play against Serena, you are playing against a fighter and a fierce competitor, but off the court, she is great to the fans and her friends and family. I think that shows a great athlete and a proper champion.

My only problem with Serena is that I’m tired of losing to her. So I told her, Watch out, I’m coming for you in 2015.​