By Clare Lovell

LONDON (Reuters) - Serena Williams battered Anastasia Pavlyuchenkova 6-4 6-4 at Wimbledon on Tuesday to join sister Venus in the semi-finals and said she was surprised and delighted the pair were still competing at the top of their sport.

The defending champion, seeking an elusive 22nd grand slam to equal Steffi Graf's open-era record, showed no sign of slowing down against 25-year-old Russian Pavlyuchenkova as they exchanged explosive groundstrokes.

Although Venus, 36, and Serena, 34, are the oldest women in the women's singles, the latter said they did not discuss retirement. They would know when the time came.

"I'm surprised at the longevity of it. That definitely took me by surprise," Serena said after the sisters also completed a 6-4 6-3 doubles victory over Czech pair Andrea Hlavackova and Lucie Hradecka following their singles matches.

"When you're younger and you have a dream and you say it and you believe it, that's one thing. But for it to really happen and to come true, it's just a completely different emotion."

Serena's experience showed on Centre Court against Pavlyuchenkova, who has never gone beyond the quarter-finals at a grand slam.

The American simply bided her time until she was able to seize on moments of weakness from the Russian to ram home her advantage.

She found a chink in her opponent's serving armory in the ninth game of the first set.

POTENT WEAPON

Pavlyuchenkova, the 21st seed, went for her shots but never found a way to counter the Williams attack. Nerves told again in the same game of the second set, where a series of errors allowed the American to break and then serve for the match.

"She was serving amazingly today," Pavlyuchenkova said. "She was there from the first point to the last."

Serena said she did not know how she had developed such a potent weapon.

"I'm not as tall as all the other players. So it's strange that I have such a strong, hard serve," she said.

"But I have to say what I think really is my game is my mental toughness because just not only to be able to play, to win, but to be able to come back when I'm down.

"Both on the court and after tough losses, just to continue to come back and continue to fight, it's something that takes a lot of tenacity."

The world No.1 -- a position she has held for more than three years -- wrapped up the victory on her second match point with a 123 mph ace for 11 in total.

(Reporting by Clare Lovell; editing by Ken Ferris)