With Andy Murray on the brink of his first semi-final appearance at the ATP World Tour Finals since 2012, we ask just how that once jittery second serve has become such a potent weapon... Murray knows victory in his second round-robin match against Nishikori on Wednesday would very likely be enough for a first semi-final appearance at Londons O2 since 2012.He came out second best to Nishikori at the US Open and knows the Japan No1, who beat Stan Wawrinka on Monday, represents a big threat. Six months half price Upgrade to Sky Sports now to watch Man Utd v Arsenal this Saturday and get the first six months half But expect to see a different Murray to the one who went down in five gruelling sets at Flushing Meadows in early September.The 29-year-old Scot has since won titles in Beijing, Shanghai, Vienna and Paris in the space of five weeks to send him to the top of the rankings. Murray wins on debut as No 1 Andy Murray beats Marin Cilic in ATP World Tour Finals opener The improvements in his game under the guidance of Amelie Mauresmo, Jamie Delgado and Ivan Lendl in 2016 have helped him refocus and implement subtle tactical changes to his game around the time of the Monte Carlo Masters - including his service action and ball toss.Since then, he enjoyed a remarkable summer by winning Wimbledon for a second time and holding on to his Olympic crown with victory in Rio. I made improvements to my second serve which then freed me up for more pace on my first serve, Murray told Sky Sports Tennis analyst Mark Petchey at the Sky Pad, on Monday.Asked about his average second serve speed increasing by six per cent in the space of 12 months, he added: It was technical. I worked on some things and changed a few things around which has made me feel more comfortable.I feel like I have more cover and control on the ball and that also allowed me to hit different serves on the second serves, rather than predominantly going to the backhand. Also having the confidence to hit a 96 mph serve close to the line, which maybe I wasnt doing last year. Andy Murray explains his match tactics to Mark Petchey after he beat Marin Cilic on Monday night In 2015, Murray won 52 per cent of his second serve points, but since May that percentage went up to 56 per cent and during the US Open it was as high as 65 per cent before his shock quarter-final exit to Nishikori.His average second serve speed in 2015 was 86 mph, whereas this year, it has shot up to 92 mph with 22 per cent unreturned. Overall, hes now winning 55 per cent of points on his second serve compared with 51 per cent last year.Thats four per cent more points on his second serve, and in the modern game, thats the key difference between winning or losing a championship match when it comes to the big points. We will get the dream final Andy Murrays match against Kei Nishikori will be huge, says Sky Sports expert Greg Rusedski Murray has not had it easy during his four tournament winning-streak either. Since the US Open, he has protected his second serve and staved off quite a few break points.Theres also plenty of variety to his game now. Last year, Murray dropped his second serve in short 44 per cent of the time, while 38 per cent were one metre inside the service box. He would only risk a deep serve 18 per cent of the time. Petchey invites Murray to guess a shot during his win against Cilic In 2016 leading up to his US Open defeat, Murray was only dropping in short serves 32 per cent of the time, with 23 per cent going deep and 45 per cent one metre inside the service box.He has also added variation and disguise to his action with 57 per cent of his second serve placement containing top spin, 34 per cent using slice while just nine per cent are flat. Andy Murray: Service Record (Pre Tour Finals) Aces 527 Double Faults 199 1st Serve Points Won 76% 2nd Serve Points Won 54% Break Points Faced 415 Break Points Saved 66% Service Games Played 947 Service Games Won 85% Total Service Points Won 67% Murrays first serve numbers on hard courts have also gone up with quicker points and unreturned serves a big difference.Pre-US Open his average speed was 115 mph, but since then its shot up three miles per hour to 118, helping him win three per cent more points. Thirty-six per cent were unreturned pre-Flushing Meadows, but thats now an impressive 42 per cent since. Murray milestones Andy Murrays greatest moments revisited ahead of the ATP World Tour Finals If the Briton continues in the same rich vein of form, few can doubt his vulnerability on second serve again, and that coveted world No 1 spot will be hugely deserving of a man who has had to make huge sacrifices to make it to the top of his game.Check our game-by-game coverage from all group matches at the ATP World Tour Finals in London - including Andy Murray v Kei Nishikori - on skysports.com/tennis, our app for mobile devices and iPad and our Twitter account @skysportstennis.Upgrade to Sky Sports now to watch Man Utd v Arsenal this Saturday and get the first six months half price! Also See: Murray wins on debut as No 1 We will get the dream final Murray: No 1 hasnt changed me WATCH: Murray at the Sky Pad Rafael Palmeiro Jersey . 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Bjorn, who had a 36-hole total of 8-under 134, made a testing six-foot putt to save par on the 16th and a birdie on the 17th before bogeying the final hole after a misjudged approach shot. American Kevin Streelman was in second place after shooting a 69. These rankings are tailored to leagues that award an additional point per reception.For Matthew Berrys rankings of quarterbacks, defenses and kickers, please see Weeek 8 Staff Rankings.dddddddddddd ' ' '