Venus Williams has edged Anastasia Pavlyuchenkova in a tight encounter to become the oldest women’s grand slam semi-finalist in 23 years.

On a day of milestones, the 36-year-old former world No.1 defeated the Russian 24th seed 6-4 7-6 (7-3) in one hour and 48 minutes to reach the last four in Melbourne for the first time since 2003.

The 13th seed will face the winner of the day’s second quarter-final, American CoCo Vandeweghe, on Thursday for a place in the title match.

“I’m so excited. It was such a hard-fought match. She never let up.

“It’s wonderful to be here. I want to go further. I’m not happy just with this, but I’m so happy to have an opportunity to go further.

“It wasn’t the easiest start to the year. I had to withdraw from the first tournament and there’s so much anxiety that goes with that.”

Lining up for her 37th grand slam quarter-final, Venus was giving away 11 years to Pavlyuchenkova.

But it was the two-time Australian Open junior champion who lost her nerve – and serve – at the most critical times.

In a see-sawing opening set, Pavlyuchenkova twice led by a break, only to falter late.

“I’m just so excited that I have an opportunity to play again.” @Venuseswilliams #AusOpen #Venus pic.twitter.com/nlmCO654vQ

— #AusOpen (@AustralianOpen) January 24, 2017

From 4-3 up, the Russian dropped serve twice in a row as Williams clubbed a backhand return winner down the line to snatch the set.

The second set was almost a carbon copy, with Pavlyuchenkova again broken from the same end while serving with a 4-3 lead.

The 2006 and 2007 girls’ champion in Melbourne, Pavlyuchenkova stood within two points of defeat in the 10th and 12th games of the second set but held firm to force a tiebreaker.

But from 3-1 up in the breaker, the Russian lost six straight points and handed Williams victory with her ninth double-fault on match point.

Venus is the oldest woman to make a grand slam semi-final since legendary compatriot Martina Navratilova at Wimbledon in 1994.

CoCo Vandeweghe roars into semi-finals

The unseeded Vandeweghe was in impressive form as she eliminated seventh seed Muguruza 6-4, 6-0 in one hour 23 minutes on Rod Laver Arena.

“I really wasn’t feeling that great out there funnily enough. I was actually quite nervous,” Vandeweghe said.

“I just tried to play my best, stay within myself and keep my patterns.”

Vandeweghe’s previous best performance at a grand slam was a quarter-final at Wimbledon in 2015. Photo: AAP.

Games went on serve to start the opening set, but in the seventh game, Vandeweghe took Muguruza to six deuces before finally claiming the break of serve to lead 4-3.

The American had to come from 0-30 down in the following game to consolidate the break at 5-3.

Two games later, Muguruza made an unforced error on set point as Vandeweghe took the opener 6-4 in 56 minutes.

Things got worse for the Spaniard when she dropped the first game of the second set on her serve, with Vandeweghe crunching another forehand winner down the line to get the break.

The American’s powerful groundstrokes were putting increasing pressure on Muguruza, and a cross-court winner gave Vandeweghe the double break for 3-0.

Growing in confidence, the unseeded 25-year-old closed in on victory with a hold and then another break for 5-0.

The match ended on Vandeweghe’s serve, with a tired Muguruza hitting a forehand low into the net.

The American’s previous best performance at a grand slam was a quarter-final appearance at Wimbledon in 2015.

– AAP