Serena Williams of the U.S celebrates a point against Amara...

Serena Williams of the U.S celebrates a point against Amara Safikovic of Switzerland during their women's singles match on day two of the Wimbledon Tennis Championships in London, Tuesday, June 28, 2016. Credit: AP

LONDON — This was not the Serena Williams the tennis world is used to seeing on Centre Court. It was still good enough for the defending Wimbledon champion to advance to the second round in straight sets.

Struggling on serve and failing to dominate against a 148th-ranked Swiss qualifier playing in her first Grand Slam tournament, Williams labored to a 6-2, 6-4 victory Tuesday over Amra Sadikovic — the 80th match win of her Wimbledon career. What looked on paper like a complete mismatch turned into a tighter contest than expected.

“I never underestimate anyone,” said Williams, who had only four aces and served five double faults. “It was a really good match but I don’t think it was tougher than I thought. It was definitely tough, but I always expect the best from everyone.”

With her mother, Oracene Price, watching from the Royal Box, the six-time Wimbledon champion jumped out to a 3-0 lead, running off 13 points in a row at one stage, defying her reputation as a slow starter.

“I started fast, that’s about it,” Williams said when asked what positives she took from her performance.

The 27-year-old Sadikovic, who decided to retire in 2014 to work at a tennis club in Basel but eventually was talked into returning to the tour, made things tricky for Williams. She took a 2-1 lead in the second set and stayed even until the final game, when she was broken while serving at 4-5.

The match ended on a successful line challenge by Williams after she hit a backhand return that was called long. After the video replay showed the ball hit the back of the line, Williams grinned and threw up her arms in triumph.

Williams, who finished with 19 winners but also had 18 unforced errors, was animated throughout the match as she tried to lift her game. “I’m always shouting at myself and pushing myself, so it’s absolutely nothing different,” she said.

Since winning last year’s title, Williams has come up short in the majors, losing in the semifinals at the U.S. Open and finals of the Australian and French Opens. She needs one more Grand Slam championship to equal Steffi Graf’s Open-era record of 22 titles.

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