Williams won 2-6, 6-2, 6-4 and struggled mightily with her fierce forehand, producing a total of 37 errors, considerably more than the 21 backhand errors she committed throughout the match.
Williams hit nine forehand groundstroke winners, but 22 errors over three sets made the match a very close encounter. Her feet were often not set, her timing was off, leaving the World No.1 reaching for the ball happened more often than we have become accustomed to seeing.
Twenty of the errors were struck standing in the deuce court and 14 were committed going crosscourt, trying to employ her primary strategy of attacking Wozniacki's forehand.
Williams ultimately won by re-grouping her game better than her opponent in the third set, but it was not a dominating or clean performance. In the end, it was more about relief to still be alive in an event she has never won rather than any excitement at advancing to the final.
Wozniacki was also inconsistent with her forehand, making 17 errors on that wing, with 13 of them occurring around the doubles alley in the deuce court where Williams focused her attacking play. SAP analytics identified that as the American's primary strategy as she directed 56% of her shots deep through the deuce court. Williams hit 82% of her groundstrokes deep past the service line which ultimately gave her an area of control and focus in baseline play.
Wozniacki was more eager to trade groundstrokes through the ad court, going backhand to backhand, where she hit 52% of her groundstrokes. Williams was also more aggressive with her court position, making contact with the ball 39% of the time inside the baseline compared to Wozniacki's 24%.
This aggressive position allowed Williams to dictate more points with better geometry of the court and gain more power by striking the ball earlier. Williams' average forehand speed was 71 mph - four mph faster than Wozniacki - and the American also enjoyed a slight edge in backhand average speed - 67 mph compared to Wozniacki's 64 mph.
The other main area of forehand concern for Williams was her return, with 15 errors coming against well placed but not overpowering serves from the Dane. Williams committed 12 of the forehand errors returning in the deuce court with four coming against comparatively weaker second serves. This is something that will need rectifying heading into the US Open as Williams also committed 15 forehand return errors in losing the Montréal semifinal against her sister, Venus.
Williams will also need to tidy up her service games as well, as she only won 51% of total service points, including just 41% (12/29) on second serves, and was broken seven times. Williams had 38% of her first serves unreturned and 16% of her second serves didn't come back in play. She did blast six aces, including three in the second set and four in the third set, helping gain some much needed free points.
Wozniacki only won 45% of her service points, performing poorly on second serves, winning only 33% (11/33) for the match. No aces and eight double faults made it a very tough day at the office on serve as she was broken eight times out of 13 service games, with seven breaks occurring in the last two sets. Williams stepped in and attacked the return with her feet as she normally does, making contact with 53% of her first serve returns inside the baseline and 100% of second serve returns standing inside the baseline.
This enabled Williams to be more aggressive on second serve returns as her average return speed was 78mph against second serves compared to 72mph against tougher first serves.
Craig O'Shannessy (@BrainGameTennis) is the leading analyst for wtatennis.com throughout the 2014 season, utilizing SAP Data & Insights to uncover the patterns and percentages that dominate the game. Visit Craig's website at www.braingametennis.com for more expert strategy analysis.