article: Serena Williams will be 34 at the end of a summer that she looks all set to enter without having been beaten on tour in 2015. She heads into Saturday's final of the Miami Open with a 17-0 record, and will surely extend that winning sequence against the admirable but somewhat limited Spaniard Carla Suarez Navarro. The somewhat scary thing about Williams is that, by her own admission, she is not even playing her best, and yet she is still ahead of the rest of the field by a country mile. Serena Williams celebrates after reaching the Miami Open final to continue her unbeaten run in 2015 . Williams was taken the distance by world No 2 Simona Halep in the semi finals in Florida . The world No 1 has a win record of 17-0 since the start of the year and looks unbeatable . The younger of the two famous siblings currently has sore knees that are restricting her practice, and she was forced into a mighty struggle in the semi-final by world No 3 Simona Halep before coming through 6-2 4-6 7-5. At least the Romanian, the WTA Tour's second best player this season, was able to lay a glove on the world No 1. Every other challenger seems to be struggling in their own way and, while there is no questioning the phenomenal and enduring quality of Williams, it also says something about the rest of the field that she will probably head into the clay court season unbeaten. 1 If she wins on Saturday she will have earned £4.33m in prize-money already this season. 2 She has actually lost this season, to Agnieszka Radwanska in January' s Hopman Cup, but the event's status means it does not count in the official records. 3 Straight after the Australian Open, Serena flew all the way to Argentina to play one match for the USA in the Fed Cup — which she needed to do to be eligible for next year's Olympic Games in Rio (she beat Maria Irigoyen 7-5, 6-0). 4 Serena has lost only five sets on tour this season, two at the Australian Open, one in Indian Wells, and two in Miami. 5 The player who has won most matches this season is Simona Halep on 26, but she has also lost twice. 'Somehow I've improvised my way to the final,' said Williams, who could have finished Halep off in straight sets. 'I'm just not at my best level right now and it's a little frustrating. I am not serving well, I'm serving at 40%, so it hasn't been my best of times.' And yet still she keeps winning, with no sign of the implosions that cost her at the Grand Slams last year apart from the US Open. Last year was an unusual one for women's tennis in that eight different players reached the finals of the Majors, but those who shared the biggest stages with Williams have, with the exception of Halep, failed to train on that much. In some cases it has been a trying season that has shown again how emotionally and physically taxing it is right at the top of the women's game. The youngest of tennis' most successful siblings drops to her knees after winning the Australian Open . Williams has won the last two grand slams at the US Open (left) and in Melbourne (right) Wiliams was forced to withdraw ahead of the semi finals at Indian Wells, the only blemish to her 2015 showings . The 33-year-old was presented with a celebratory cake in Miami after winning her 700th career match . The exceptionally talented Wimbledon champion Petra Kvitova has actually sat out the last two significant events, here and at Indian Wells, with exhaustion. Happily it appears likely that she will be returning for the clay court season. Eugenie Bouchard faces the intimidating task of backing up her remarkable breakthrough year of 2014 that few expected, and made a good start at the Australian Open by making the quarter final. Yet she was in tears at Indian Wells as she lost to world No 85 Lesia Tsurenko and afterwards admitted she needed to relax more. In Miami she lost her opening match to Germany's Tatjana Maria, the world No 113 who is the highest ranked mother on the tour. America's Madison Keys, universally tipped as a future Grand Slam champion, was another one who struggled to contain her emotions here when she went out to Sloane Stephens. Of last year's other Grand Slam champions Li Na has retired while Maria Sharapova has yet to build on the strong start that she had in Australia. The world No 2 lost her opening match in Miami to Russian wildcard Daria Gavrilova and it is, in any event, difficult to build up any rivalry between her and Williams as she has lost against her the last sixteen times they have played. Spain's Carla Suarez Navarro is unlikely to provide much resistance against Williams in Saturday's final . Halep is closest to challenging Williams' dominance while Maria Sharapova needs to push on after a good start . Golden girl Eugenie Bouchard is a possible contender to challenge Williams after breaking through last year . America's Madison Keys, universally tipped as a future Grand Slam champion, was another one who struggled to contain her emotions here when she went out to Sloane Stephens. Of last year's other Grand Slam champions, Li Na has retired while Maria Sharapova has yet to build on the strong start that she had in Australia. Romania's Halep, a relatively late developer whose game is based on her supremely athletic movement, right now looks the biggest threat to Williams, and she might well be the best long-term bet for the French Open.
highlights: World No 1 has not been beaten since the start of the year with 17-0 record .
  Serena Williams plays Carla Suarez Navarro in the Miami Open final .
  Players including Simona Halep, Maria Sharapova and Eugenie Bouchard have failed to mount a serious challenge to the American veteran .
prediction: Serena Williams will 
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