LONDON—Defending champion Novak Djokovic has secured his spot in the semifinals of the ATP World Tour Finals, backing up his win over Roger Federer by defeating Juan Martin del Potro, 6-3, 3-6, 6-3, leaving his opponents to grapple for the second spot on Saturday.

It wasn’t a classic encounter; more a series of soloes than a symphony. After three unforced errors in the first four points of the match left Djokovic facing two break points, the Serb quickly produced his first service and backhand winners of the day and dominated the set from there. Del Potro produced one easy hold, but was forced to save three break points at 1-2. He was rapidly struggling to find his footing in all senses, spasmodic in attack, neglecting to capitalize on some opportunities, snatching over-eagerly at others. It’s not a recipe for success against Djokovic, who knew exactly what he had to do: Soak up del Potro’s groundstrokes, break up his rhythm with slice, and work his way inside the baseline wherever possible.

It’s not rocket science, but the aplomb with which Djokovic executed in the first set helps to explain his 10-3 dominance over the Argentine in their head-to-head. Although del Potro fended off one more break point at 2-3, when a sequence of line-grazing, luck-pushing groundstrokes from Djokovic pushed del Potro further and further off-balance—until he lunged and missed to give up the break—it felt entirely in the run of play.

Djokovic sealed the set with a second-serve ace, but del Potro returned to the court with a suddenly much-improved first serve—88 percent for the second set—and more bite to his groundstrokes. He opened the set with a storming forehand winner, then managed for the first time to get Djokovic chasing vainly from side-to-side. Even that rare, shy beast—his backhand down the line—made a couple of appearances. In turn, Djokovic seemed to be fighting off a lapse of concentration as much as his improved opponent and lost the battle with both at 2-3. While del Potro’s break was the result of an obliging drop off the netcord, it was really only a formality after a rash of Djokovic unforced errors.

At 2-5, del Potro slapped forehand returns out of the court, trying to get some adrenaline out of his arm before he had to serve for the set. It worked, but the clay had started to seep back into his feet in that last game and Djokovic was reviving.

Del Potro would have one real chance in the third to take control of the set: At 1-1 on Djokovic’s serve with a second break point, Nole at net, and a running forehand pass just about there to be made. It wasn’t a bad miss, but del Potro’s reaction would suggest otherwise, as would the serious thought he gave to smashing his racquet after Djokovic held.

Del Potro’s Eeyore-like demeanor makes it tempting to ascribe a sort of fatalism to him, but while that’s probably going too far, the depth of frustration he exhibited when he was still very much in the match implicitly shifts the focus to the question of his mental resilience. He missed another passing shot in Djokovic’s next service game, couldn’t get near a break, and at 2-3—the fatal moment in each set—made four genuinely poor unforced errors to be broken to love. Djokovic sailed serenely to the victory.