Image source, AP

A delicious lob, disguised beautifully, from Williams hauls the American back to 30-30. An 122mph ace - her fastest serve so far - brings up game point but Keys forces deuce with a backhand winner down the line.

But Williams clings on to win the next two points and seal the game. Keys to serve to stay in the set once more.

Keys finds herself under a bit of pressure as Williams closes to 30-40, but deals with it well, firing an ace to seal the game and level the set.

Every time Serena Williams has got past the quarter-finals in the Australian Open, she has gone on to win the title. The top seed puts herself into the driving seat to continue that streak with another solid service hold. You get the sense Williams is starting to get into her stride now...

Piers Newbery
BBC Sport at Melbourne Park

"All eyes in the media seats are trained on Keys' heavily strapped left thigh, with the odd murmur that we are starting to see signs of discomfort from the teenager. If she can keep getting that first serve in then she has a chance, having won 91% of the points behind it so far, but miss that and she's in trouble, winning a woeful 17% behind the second serve."

Impressive response from Keys, the young American putting the disappointing of being broken back behind her with a comfortable service hold.

Image source, AP

Williams races through her service game to love to move ahead for the first time in the match.

Madison Keys has looked like a world-class player for 15 minutes but the American shows her first sign that she can feel that left injury. She winces as she lands on it after a first serve.

Suddenly, she's facing two break points. Williams sends a tricky volley long to miss out on the first but is rewarded with the second as Key's backhand breaks down. Williams breaks.

A better start from Williams in this service game, the American opening with a winner before cruising to 40-0. Keys hits back in the next two points but then fires wide to concede the game.

Keys' hitting heavy off both flanks at the moment and her serve is working beautifully, too. The result is another easy service game.

Williams, who has been struggling with a cold for a few days now, doesn't look to quick on her feet at the moment. But nonetheless, the top seed summons some fine first serves and closes out her service game with an ace to get on the board.

Keys' left thigh is heavily strapped, the American requiring a medical timeout during her victory over Venus Williams in the previous round. But it doesn't look like it's causing her any problems as she races through her first service game to consolidate the break.

Madison Keys is playing in her first Grand Slam semi-final - but you wouldn't know that based on her start in this game. The 19-year-old batters the Williams serve early on to carve out an opening at 0-30.

Williams recovers to 30-30 with an ace, but gifts her opponent a break point with a backhand error and then throws in another to concede the game. Keys off to a flier.

Piers Newbery
BBC Sport at Melbourne Park

"Madison Keys steps into the Grand Slam spotlight for real today, but she made her first breakthrough in the more sedate surroundings of Eastbourne last summer. The teenager won her first WTA title at Devonshire Park, the perfect place to get some experience of playing tennis in the wind. It was also where she picked up the thigh injury that forced her to retire at Wimbledon, and resurfaced in her semi-final win over Venus Williams on Tuesday."

The warm-up is over. Serena Williams will be the first to serve. Let's play tennis.

Image source, Reuters

Madison Keys, 19, is going for a rare Williams-Williams double, taking on Serena after beating Venus in the quarter-finals. If she achieves the feat, she will be only the eighth player to beat both Williams sisters at the same tournament.

Right, then. Ready for a second semi-final? Good, because we've got one coming up. After an all-Russian encounter, we have an all-American match-up - top seed Serena Williams v unseeded Madison Keys.

Image source, Getty Images

Victory for Sharapova keeps alive her hopes of snatching the number one ranking from Serena Williams. The Russian needs to win the Australian Open and hope Williams loses to Madison Keys.

Piers Newbery
BBC Sport at Melbourne Park

"Coach Sven Groeneveld is up on his feet along with the rest of Team Maria, decked out in blue tops and baseball caps and looking not unlike a patrol of UN peacekeepers. The Dutchman has certainly played a big part in keeping Sharapova at the very top of the sport after serious shoulder problems. 'She's not known for her defence but it's improved so much,' says 18-time Grand Slam champion Chris Evert on ESPN. 'I never thought I'd see the day when Maria Sharapova was flying around the baseline.'"

Sharapova races to 40-15 and then watches on as Makarova's backhand lets her down again. The Russian second seed and 2008 Australian Open champion is through to her fourth final at Melbourne Park. She'll play Serena Williams or Madison Keys, who will be on-court shortly in the second semi-final.

Makarova scrapes through her service game for only her second hold of the set, but you get the feeling it is only delaying the inevitable. Sharapova to serve for a place in the final.