Cobolli-Blockx: the third round is decided on the Belgian's first serve

Alexander Blockx has a serve that, when the first one lands, doesn't get played. At Cincinnati, less than three weeks ago, that serve wasn't enough: he lost to Flavio Cobolli 7-5, 4-6, 7-5 after serving for the match at 5-3 in the third set. Today on Louis Armstrong the same match gets replayed, and it lives in the same place: how many first serves the Belgian lands when it counts.

The service threshold, and what happens below it

The Belgian is 1.93m tall and his service model is built on risk: a flat first serve at full speed, with little safety net underneath. When the shot works, his service games last four points. When it jams, Blockx ends up defending second serve after second serve, and the gap in the baseline rallies starts to weigh in full again.

The New York tournament has already shown both versions. Against Tomás Barrios Vera, a first round closed out 6-3, 6-4, 6-2, the serve held up and the match was never in question. Against Marco Trungelliti, three days later, the serve misfired and he dropped a set (6-3, 6-2, 3-6, 6-1) against a player who, on paper, shouldn't have taken one.

Cobolli doesn't need to attack the first serve. He has to stay alive in the games where it lands and show up on the second, where his early, inside-the-court return makes sense. Over best of five sets that plan pays for itself, because the longer the match goes, the more second serves arrive.

What happened at 5-3 in the third at Cincinnati

There's one number from that match that weighs more than any average percentage. Blockx was serving for the match at 5-3 in the third and lost the last four games. At the moment he needed to close it out, the serve stopped holding him up, and Cobolli won without changing a thing about his plan.

It would be convenient to call it a structural flaw, and it would be wrong. Back in April, at Monte Carlo, Blockx was a qualifier and beat Cobolli in two sets after eliminating Shapovalov, on a surface that in theory favors the returner. The 1-1 head-to-head should be read that way: two different days, not two different surfaces. Over best of five sets, though, the number of times Blockx has to close it out doubles.

Counting the hours

Cobolli reaches the third round with nine sets in his legs. Four hours and twelve minutes in the first round to beat Francisco Comesaña 3-6, 2-6, 6-3, 6-4, 6-4, with an ankle problem that flared up early in the match according to Ubitennis. Then another two hours and forty-one minutes against Tristan Schoolkate, again coming back from a set down. Blockx has played seven sets.

The slow starts matter more than the accumulated fatigue. Cobolli has lost the first set in both matches of this tournament. Against a server who can steal a set with fifteen minutes of grace, giving away the first set means having to win three of the next four against an opponent who's serving well.

The prediction, and what could prove it wrong

A slight edge for Cobolli, nothing more. The draw says fifth seed against 28th seed, the prediction models treat the match as little more than a coin flip, and they're right. The Italian wins if the match runs long, because every extra set increases the number of Blockx second serves he has to face and stretches out the tally of games the Belgian has to hold.

The condition that flips everything is measured live and needs no statistics. If Blockx closes out the second set without ever facing a break point, the match is his: at that point Cobolli is chasing it with nearly seven hours from his first two rounds already in his legs. If instead the Belgian starts stringing together two second serves in a row, the chances for the Italian show up on their own, just like at Cincinnati.