Serena Williams does not believe she has hit her top form at Wimbledon despite ousting defending champion Petra Kvitova in straight sets on Tuesday.
Williams, in the hunt for her fifth title in SW19, turned on the style on her debut under the Centre Court roof to out-muscle the Czech in a 6-3 7-5 win.
The American blasted the 22-year-old aside with an impressive service game but insisted she can still ramp it up a notch in the semi-finals, where she will face second seed Victoria Azarenka.
"It just feels like a good win," Williams said after the match.
"You know, she was playing very well. I don't know if it was more about dethroning (Kvitova) or going out there and playing the match and doing the best that I could.
"I feel like I've been returning really well in practice, much better than any of my matches, so I'm thinking one day it's going to come through.
"I can still return a lot better, but it just hasn't quite clicked yet. But today was definitely better than the other matches."
Azarenka saw off Tamira Paszek thanks to a second-set tie break after the Austrian fired long on match point to win 6-3 7-6 (7/4).
"The games were really close," Azarenka said after the match. "At the beginning of the second set we were really holding our serves well. It was a close match and there were a lot of long rallies but I'm glad I could manage to win.
"It's a great feeling to be in the semi-final of a grand slam but I still want to go all the way and it still seems so far away.
"There's no better opponent than Serena in the semi-finals so I'm really looking forward to that match."
Eighth seed Angelique Kerber booked her place in the last four with a hard-fought win over fellow German Sabine Lisicki.
Kerber claimed a 6-3 6-7 (9/7) 7-5 win over Maria Sharapova's conqueror after two and a half hours to reach her first Wimbledon semi-final. Kerber looked like she had blown her chance when she missed three match points in the second set and then trailed 5-3 in the decider but Lisicki faltered when serving for the match and Kerber reeled off four games in a row to triumph.
"It's unbelievable, the match was so tough," Kerber said. "Suddenly she played unbelievable tennis. I'm very happy to be in the semis here for the first time.
"We have known each other from a young age and it was mentally not easy, but it doesn't matter who you play, you have to play your best and I stuck to my plan and played a very good match I think."
In the other rain-affected quarter-final, Agnieszka Radwanska and Maria Kirilenko finished off the last few games of their match under the roof on Centre Court.
The match was suspended on Court One due to failing light at 1945, with Kirilenko serving at 4-4 in the third set.
Radwanska powered ahead after the break to win 5-7 6-4 7-5 to set up a semi-final clash against Kerber.
The Polish number three seed will become world number one if she wins the title and rival Azarenka is knocked out at the semi-final stage.