This week the top players in the women's game have been fighting it out in the Tour Finals.

This year's tournament promised to be pretty open with World number 1, Serena Williams withdrawing. The American has struggled with her fitness this season and the disappointment of not completing the calendar Grand Slam (holding all 4 Grand Slam titles in the same calendar year) prompted her to call an early end to her season.

Sharapova had not completed a match since losing in the Wimbledon semi final. She played once in Wuhan last month but retired injured in the third set against Barbora Strycova. She wasn't expected to be able to produce her best tennis this week.

The line up for the finals was not confirmed until the last minute as several players were in with a chance of making it to Singapore.

One of the surprises from the Round Robin matches was the form of Sharapova. She was expected to be rusty without any recent match practise but right from the start of her first match she was sharp. Her serve, often her Achilles heel, has been pretty reliable too. She lost the opening set to Radwanska in her first match but came through to win a tough three setter.

Straight sets wins over top seed Halep and US Open champion Pennetta saw her cruise into the semi finals on Saturday.

Halep was disappointing after destroying a tired Pennetta in her opening match. Her 6-4, 6-4 score line against Sharapo

va flatters her as she was outplayed by the Russian. Against Radwanska she was 5-1 up in the first set tie break but lost the next 6 points. She showed no fight in the second set and having won the opening game she lost the next 6.

Radwanska needed just one set from her match with Halep to give herself a chance to reach the semi finals. She easily managed that and other results went her way. She will progress to the latter stages. She has responded well to the pressure of qualification requirements but there is a chance she has already given all she has in terms of energy.

Pennetta had an awful start against Halep and won just 3 games but was more competitive after that. She had her chances against Sharapova in their opening set but the Russian ran away with the second set. The Italian, playing her last tournament before retiring, did manage a win over Radwanska but finished at the bottom of the Red group.

Muguruza was top seed in this group. The Wimbledon finalist has really matured this season. Reaching a Grand Slam final at the age of 21 has given her a real boost of self belief. Before this season she would get into good positions in matches but would fall apart mentally far too often.

This season she has moved her game forward and is now an excellent big game player. She has won all 3 of her Round Robin matches, dropping just 1 set to Kvitova. She is also in the semi finals of the doubles tournament with Suarez Navarro and looked a little tired at the end of the second set with Kvitova. She fought back from being broken in the third set to take it 7-5.

Kvitova has been playing with Mononucleosis this season though her recent medical tests proved to be negative. In her first match with Kerber she started badly and having lost the opening set 6-2, she didn't realise the set was over and was composing herself for the next game at the back of the court. The umpire had to tell her the set was over! She said in her press conference that she felt "weird" during the match.

She played much better against fellow Czech Safarova and won in 2 tight sets 7-5, 7-5. She had her chances against Muguruza. Before the match she needed to win a set to give herself a chance of reaching the semi finals. She took the second set and thanks to Safarova beating Kerber in straight sets in the final group match, she will go forward to Saturday's semi final.

Kerber had never won a match at a Tour Finals but thanks to her win over Kvitova in her first match, she needed just 1 set from her match with Safarova to reach the semi finals. Kerber so often chokes the big matches and this was no exception. Safarova played a great match but Kerber made too many errors. The situation got to her and she lost in 2 sets.

They have played 6 times with Muguruza leading 4-2. She has won their last 4 meetings. All 4 of her wins came in 2015. Three of the 4 matches this season went to 3 sets. Three of the matches were on hard courts, though none of them were indoors.

Muguruza obviously goes into this match as clear favourite. She has played some breathtaking tennis this week. Her confidence is sky high but as we saw against Kvitova if things start to go against her she can start making more errors.

Radwanska will be no pushover though. She had a bad period of form earlier in the season but ended the year strongly. She was competitive in all her Round Robin matches despite losing twice. She was the only person to take a set from tournament favourite Sharapova. She as amazing shot making abilities and has a very sharp tennis mind. She could outwit the less experienced Muguruza.

Muguruza is still only 22. She is also in the doubles semi final as I said earlier. I think if she gets into a winning position she will get a little tight.

Radwanska's best hope of success is that the situation and perhaps a little fatigue gets to Muguruza. She will move the young Spaniard all over the court and make her work for every point.

I can see this being a tight encounter and 3 sets would not be a surprise but I have to go with Muguruza to progress to the final.

They have met 9 times with Sharapova leading 6-3. One of Kvitova's wins was due to Sharapova retiring back in 2011.

The last time they played was at last year's Tour Finals. It was in the Round Robin stage and Kvitova won in straight sets.

Kvitova's form has improved with each match though her fans will be concerned at the number of time she was broken in her last match. She was broken 8 times by Muguruza.

Sharapova hit too many double faults in her first two matches but only one against Pennetta. When she is in this sort of form she can be hard to beat.

Kvitova will need to do a much better job from the service line if she is to get anything from this match. If she can protect her serve more regularly then she can push Sharapova very close.

As with the other semi final I think we have a good chance of seeing three sets but I am going with Sharapova to take her place in Sunday's final.