Tramlines and Wide Groundstrokes

The NextGen Finals are played on an unusual court, in that the surface is marked only for singles matches, leaving out the “tramlines” that define the doubles alleys. Virtually all tennis events includes doubles, as well, so this is rarely an option. The ATP has skipped tramlines at season-ending events before, but at the end of the 2010s, the singles-only court is exclusive to the NextGen Finals.

One might reasonably wonder whether the unique paint job has any effect on play:

https://twitter.com/furryyelloballs/status/1192168870133583877

I discussed this on a recent podcast with Erik Jonsson, and we tentatively concluded that tennis pros (even young ones) with thousands of hours of playing experience shouldn’t be affected by a tweak to the appearance of the court. But why speculate when we can look at some data?

The Match Charting Project, my volunteer-driven effort to log shot-by-shot records of professional tennis matches, notes various details about errors–forced or unforced, and “type”–net, deep, wide, or wide-and-deep. MCP contributors didn’t immediately take to the NextGen Finals–before this week, the 2018 final was the only charted match out of the 6,600 matches in the dataset–but 2019 was different. We now have shot-by-shot stats for 8 of the 15 matches played in Milan last week. (Big thanks to Carrie, who took charge of Alex de Minaur’s entire run to the final.)

Quantifying wide errors

We’re interested in the frequency of wide errors, which isn’t quite as simple as it sounds. I chose to focus only groundstrokes, and I also excluded forced errors–shots on which the player might not have much control of the direction of the ball.

Here are three metrics we could use for the frequency of wide errors:

  • Wide errors per point
  • Wide errors per unforced error
  • Wide errors per “makeable” groundstroke–that is, groundstrokes that were either unforced errors or put in play

Wide errors per point is probably too crude, but it does have the advantage of simplicity. Wide errors per unforced error might have some value, telling us in what direction a player was most aggressive. The last, wide errors per makeable groundstroke, is probably the best representation of what we’re looking for, as it tells us how frequently a player tried to hit a shot and it went wide.

Here are de Minaur’s numbers for his five 2019 NextGen matches, along with his hard-court aggregates from 28 other charted matches in the last two years:

          Wide / Pt  Wide / UFE  Wide / GS  
NextGen        2.7%        1.5%      21.7%  
ATP Hard       3.0%        1.4%      21.4%

At least for Alex, the tramlines don’t seem to make much of a difference.

Let’s look at the slightly larger group of players. We have eight matches, which means 16 records of one match for a single player, including at least one for each of the eight guys who qualified for Milan. Here are the three wide-error rates for the NextGen Finals matches, along with the same players’ wide-error rates for other charted hard court matches in the last two years:

          Wide / Pt  Wide / UFE  Wide / GS  
NextGen        3.2%        1.8%      19.5%  
ATP Hard       3.2%        1.8%      23.1%

For our first two metrics, there is absolutely no effect. Tramlines or no tramlines, wide errors mark the end of 3.2% of points, and 1.8% of total unforced errors. (The 3.2% figure is per player, meaning that 6.4% of points were ended with a wide error.)

The third metric, though, is more interesting. On tour, these players make a wide error on 23.1% of their “makeable” groundstrokes. That number dropped by more than one-seventh, to 19.5%, on the tramline-free court in Milan. At the same time, the overall rate of unforced errors (not just wide errors) increased compared to the same players’ efforts on hard courts at other events.

Deep mind

I see two possible explanations for such a substantial drop. First, we don’t have much data, and maybe it’s just a fluke of a small sample. Some of the difference can be traced to Ugo Humbert, who didn’t make a single wide error in his one charted NextGen Finals match. (Humbert’s usual wide-error rates are close to average.) Without a lot more matches played on tramline-free surfaces–not to mention charts of those matches–we won’t be able to draw a firm conclusion.

Second, it could be a real effect stemming from some aspect of the conditions in Milan. The lack of tramlines really might, as Lisa puts it, “focus the mind.”

Compared to other innovations trialed at the NextGen Finals, the singles-only court gets very little press. But unlike, say, the towel rack or the shot clock, it might just have a small effect on play.