US player Venus Williams plays a shot during her first...

US player Venus Williams plays a shot during her first round women's singles defeat to Russia's Elena Vesnina on the first day of the 2012 Wimbledon Championships tennis tournament at the All England Tennis Club in Wimbledon, southwest London. (June 25, 2012) Credit: Getty Images

WIMBLEDON, England -- She called herself a "great player," and Venus Williams was, especially at Wimbledon, where she has won five championships. But Monday, for the first time since her 1997 debut at the All England Club, she was an opening-round loser.

In only her sixth tournament since being diagnosed with the autoimmune disease Sjogren's syndrome -- which saps her strength -- last August during the U.S. Open, Williams was beaten, 6-1, 6-3, by Elena Vesnina of Russia.

Broken the first three times she served, Williams fell behind 5-0 in the opening set and never recovered. But when she was asked after the match whether she has reached the end of a magnificent career, she was defiant.

"I feel like I'm a great player," said Williams, who turned 32 last week. "I am a great player. Unfortunately, I have to deal with circumstances that people don't have to deal with normally in a sport, but I can't be discouraged by that. I'm up for challenges. I have great tennis in me. I just need the opportunity."

Williams said she tries to stay positive and focused.

"There's no way I'm going to sit down and give up just because I have a hard time the first five or six tournaments back," she insisted. "I'm not going to let anything get to me, like crazy questions. But I'm tough. Let me tell you. Tough as nails."

A seven-time major singles winner, Williams returned to tournament play in March in an attempt to raise her ranking to qualify for the U.S. team at the London Olympics. She was successful. The tennis competition will be at Wimbledon.

"I'm really proud of my efforts to get my rankings up for the Olympics," Williams said. "That's one of the toughest things I've ever done in my life. Yeah, I came back early. Do I think I'm paying for it now? . . . I really don't know."

Williams gave credit to Vesnina, 25, who unlike Venus played a couple of grass-court warm-up tournaments. Venus had not played since a second-round loss to Agnieszka Radwanska in the French Open 41/2 weeks ago.

"She's been on the tour a while,'' Venus said of Vesnina, ranked 79th to Williams' 58th. "So players like her, they know how to hold on to a game, hold serve. She played well.''

Isner falls in five sets. Two years ago, John Isner played that historic three-day, 183-game match, beating Nicolas Mahut. On Monday, Isner, seeded 11th -- second highest among American males -- was upset by 73rd-ranked Alejandro Falla of Colombia, 6-4, 6-7 (7-9), 3-6, 7-6 (9-7), 7-5. Americans Ryan Harrison, Ryan Sweeting, Michael Russell, Jamie Hampton and Sloane Stephens all won, and James Blake, Donald Young, Melanie Oudin and Vania King all lost.

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