The seven-times grand slam champion was leading 4-1 in the second set when play was halted for about eight minutes to allow 19-year-old Keys to have treatment on her left thigh.

When play resumed, Williams closed out the set but went on to lose the match 6-3 4-6 6-4 at Rod Laver Arena. "I felt like I really got off rhythm after that break. It was hard to come back," the 34-year-old told reporters.

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"Never really felt like I got back to that rhythm that I was building at 4-1 and really just started to feel good about everything.

"I think for her, when you're injured, you feel a little tentative at the beginning. After a while you feel okay and so then you start to be more relaxed."

Williams broke Keys to take a 4-3 lead in the deciding set but was broken back immediately and her hard-hitting opponent won three straight games to roar to victory.

Williams' quarter-final appearance was her best performance at a grand slam since the semi-finals of the 2010 U.S. Open and continued her outstanding start to the season after beating Caroline Wozniacki to win the Auckland title.

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Her ranking will improve to just outside the top 10 when the new standings come out, an impressive return after her struggles in recent years with Sjogren's syndrome, an auto-immune disease that can cause chronic fatigue and joint pain.

"It's definitely been a great last four weeks, I think," Williams said.

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"A lot of match wins, a lot of momentum, just a lot of great things happening that everybody wants to happen in their career. "I think a lot of players would have been happy with this tournament. I'm happy with progress. I'm not happy with a loss, I'll tell you that."

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