The 24-year-old, who has yet to win a Grand Slam title, was in stunning form as he defeated Russian Karen Khachanov 6-3, 6-1 to become the first German man to win a singles Olympic gold.
Becomes only third Indian to win a singles match at the Games.
Sumit Nagal was blown away by world number two Daniil Medvedev's thunderous strokes and serves in the singles second round as India's tennis challenge all but ended at the Tokyo Olympics on Monday.
The Indian pair look as if they would cakewalk through the contest after breezing through the first set. However, the Kichenok sisters rallied splendidly from the dead to pull off a 0-6, 7-6(0,) 10-8 victory on court 11 of the Ariake Tennis Centre.
A look at how India's athletes fared at the Tokyo Olympics on Saturday.
Novak Djokovic eased into the last four with a commanding performance over Japan's Kei Nishikori but world number two Daniil Medvedev bowed out of the men's singles at the Tokyo Olympics.
Novak Djokovic launched his bid to stay on course for a Golden Slam with a clinical 6-2, 6-2 win over 139th-ranked Hugo Dellien of Bolivia on Saturday, as sweltering conditions in Tokyo took a toll on the players on the opening day of tennis at the Games.
Pint-sized 13-year-old Momiji Nishiya took gold in street skateboarding -- new to the Olympics in Tokyo -- while in one of the Games's staple sports a "Terminator" hunted down a dominator to set the swimming pool alight.
Former World No 1 Ana Ivanovic brushed aside Caroline Wozniacki to win the Pan Pacific Open in Tokyo on Sunday and move closer to sealing a berth for the season-ending WTA finals in Singapore.
