Djokovic took out No. 7-seeded Tomas Berdych 7-6 (7-5), 6-4, 6-3 just moments after Del Potro, seeded eighth, finished off a 6-2, 6-4, 7-6 (7-5) win over fourth-seeded David Ferrer.
They will meet in Friday's semifinals. Djokovic, seeking a second Wimbledon title, is 8-3 all-time versus del Potro, although del Potro won their most recent meeting.
Janowicz, the No. 24 seed, defeated his Polish countryman Lukasz Kubot 7-5, 6-4, 6-4. The fourth semifinalist was to be either No. 2-seeded Andy Murray or unseeded Fernando Verdasco, who were in the fourth set with Verdasco winning the first two, when Janowicz completed his win.
Djokovic rallied from a two-break deficit in the second set and won six of seven games to take the set and command of the match. Djokovic cashed in the only break opportunity of the third set and earned his fourth consecutive trip to the Wimbledon semifinals.
Djokovic allowed Berdych only two break opportunities -- those allowed Berdych to take the second-set lead -- while finishing with four breaks himself. Djokovic won more than 70 percent of the points on serve.
Del Potro seemed to injure his knee in the first game against Ferrer but relied on a superior serving game to gain his third Grand Slam semifinal. He won the U.S. Open in 2009 and was also a semifinalist at the French Open that year.
To get to the 2013 Wimbledon semis del Potro saved the only two break points he faced and won three-quarters of the points when serving. Del Potro had three breaks but there were no break chances in the third set. Del Potro, however, closed the match with two of his 42 winners.
Janowicz fired off 30 aces against Kubot and didn't allow Kubot to convert any of his six break chances while getting one break late in each set.