Keys injury time-out cost me momentum, says beaten Venus

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.

"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.

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.

"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."