Widjaja, ranked 21st in the world juniors, needed only less than 50 minutes to turn back Szili, who booted out seventh pick Megha Vakaria of India in Wednesdays round-of-16, and set a meeting with No. 4 Amber Liu of the United States, winner over Jung Yoo-mi of Korea, 3-6, 7-5, 6-1.
Also making it into the semifinals were second seed Hsieh Su-Wei of Chinese Taipei and No. 3 Sanja Mirza of Indonesia.
Hsieh, 28th in the girls 18-and-under world ranking, dumped Pishaya Laosirichon of Thailand, 6-1, 6-2, while Mirza downed Alexandra McGoodwin of the United States, 6-4, 6-4, to ensure an all-Asian semis.
Boys favorites Jonathan Chu and Prakash Amritraj, son of the 70s tennis great Vijay, led the American charge as they pulled off contrasting triumphs to barge into the semis.
No. 1 Chu, 46th in the world boys ranking, repulsed Pawei Dilaj of Poland, 6-1, 6-3, while the second ranked Amritraj bumped off another Polish, Michael Przysieny, 6-2, 6-2.
Australian Clinton Letcher, whose father Cliff was Amritrajs doubles partner in the 70s, and Slovenian Luka Gregorc also came up with wins to clinch the last semis berths.
Letcher, 18, responsible for No. 4 Indonesian Kumar Sipaeyas ouster, defeated Scott Oudsema of the USA, who retired in the second set due to foot blisters after trailing, 4-8, 0-1, while Gregorc, who owned fifth seed American Jason Zimmermans scalp, outlasted Yu Hiu Tung of Hong Kong, 6-2, 3-6, 6-3. Joey Villar