Williams has yet to drop a set in this year's tournament
Defending champion Venus Williams remains on course for a fifth Wimbledon title after beating Thailand's Tamarine Tanasugarn to reach the semi-finals.
The American, 28, was not at her best but still had too much for Grand Slam quarter-final debutante Tanasugarn, 31, winning 6-4 6-3 on Court One.
Williams next plays fifth seed Elena Dementieva, who beat Nadia Petrova 6-1 6-7 (6-8) 6-3 on Centre Court.
Dementieva led by a set and 5-1 but was pegged back and needed a third set.
An all-Williams final remains a possibility after Serena Williams thrashed Agnieszka Radwanska later on Tuesday and she will face Chinese wildcard Jie Zheng in the other semi-final.
"It would be amazing if we were both in the final," Williams told BBC Sport afterwards.
"I am in the semis so I have just got to take it one more step now and try to keep going. I have to keep my serve as it is, that is the key."
I felt so tight when I served for the match in the second set
The four-time champion appeared to be limping at the end but played down any injury fears.
"My hamstrings are a little tight but that is to be expected when you have played the number of matches I have in singles and doubles," she said. "I'll be fine."
Williams never looked in danger of falling victim to an upset on Court One, following a week of shocks at the All England Club, but nor did she really hit the heights.
After breaking serve in game three she was immediately broken back, and even after edging ahead again in the fifth game Williams could not shake off the dogged Tanasugarn.
It was a similar story in the second set, which saw Williams break at the start but face break points in the sixth game, and it was the weak Tanasugarn serve that prevented the Thai from ever really threatening.
Dementieva is into the last four at Wimbledon for the first time
Tanasugarn had no complaints - but did admit she wished she was taller.
"I was joking to myself, I have so many break points in the first set, but Venus doesn't give you any easy chances... she was acing me many times.
"In my next life I want to be tall as her, please.
"I tried to be aggressive and step up but she just served very well. That's the ability of great players."
Over on Centre Court, the two Russians played out a bizarre match in which Petrova collapsed after an early line call to lose six consecutive games, before Dementieva conspired to let a 6-1 5-1 lead disappear.
It was Dementieva - the highest remaining seed in the draw - who had the edge for most of the match but, as so often, nerves got to the fifth seed when the end was in sight.
But she gathered herself at the start of the decider to take a 4-0 lead and, despite a predictable wobble when she dropped serve, Dementieva just about got over the line.
"I am extremely happy for the first time in my life to be in the semi-finals here," Dementieva told BBC Sport.
"It is a great feeling and was a big pleasure to play on Centre Court, although it was a difficult match. I feel really happy.
"I felt so tight when I served for the match in the second set. I was so close to finishing in two sets and don't know what happened.
"Maybe I was thinking about the French Open quarter-final. It was so difficult to play in the beginning of the third set but I tried to stay positive.
"It does not really feel like Wimbledon, there has not been much rain this week and it was extremely hard during the match because of the heat. I feel exhausted right now."
Petrova had been the first of the pair to struggle when a line judge called her for crossing the middle service line while serving in game four.
It was a dubious decision at best but the Russian was clearly upset and promptly dropped serve, and the next five games in a row.
"The linesman was not really paying attention to the right things. I've never been foot-faulted in this way," she said.
Petrova's anger provided a comical moment
"He said my right foot was on the T line, but I didn't let the ball go. That wasn't the start of the motion.
"It was a very important game. After losing that I gave her a lead of 4-1 so it was difficult to catch up."
The 21st seed was also unhappy that the players were not better protected from the sultry conditions during the changeovers.
"I thought that today was very hot weather conditions, and I was very surprised that we were not treated as we should be.
"I think for the first two sets there was not a ball-boy putting the umbrella behind us. I think the temperature reached up to 27C, and it was very hot. Being in the stadium I think makes it even hotter."
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