TOKYO, July 27 (Reuters) - Kei Nishikori kept Japan"s tennis medal hopes alive with a tough, three-set victory in the men"s singles on Tuesday, hours after compatriot Naomi Osaka was eliminated from the women"s event in one of the biggest upsets of the Tokyo Games. Nishikori, who won the bronze medal at the Rio Games in 2016, said he was feeling confident on the court though admitted that it was tough playing at home with crowds absent from matches amid strict pandemic restrictions in the capital. "I only come in here and I don"t see any spectators. It"s really tough to feel home but I have a lot of support with my team, you know. That"s the only good thing, maybe," he said after his 7-6(5) 3-6 6-1 win over Marcos Giron of the United States in the second round. "But I think I really enjoy being on the court, because I"m playing good and this is something I was dreaming of from, you know, when I was little... this is a great, great week I"m having. I"m really enjoying this week." Nishikori won a close tiebreak over the big-hitting American in the first set but could not recover after going down an early break in the second, taking the match to a decider. The 31-year-old stepped up his game in the final set, breaking Giron"s serve three times to set up a meeting with Ilya Ivashka of Belarus. Nishikori, who beat fifth seed Andrey Rublev in the opening round, is ranked 69th in the world but has long been the top Asian player on the men"s tour, having reached a career-high fourth shortly after his run to the U.S. Open final in 2014. Play was briefly suspended at Ariake Tennis Park on Tuesday due to a morning downpour, although the day"s marquee matches went ahead as scheduled with the roof closed on Center Court. World number four Stefanos Tsitsipas of Greece avenged his straight-sets defeat by Frances Tiafoe at Wimbledon last month, taking out the American 6-3 6-4. Tsitsipas attributed his win to improved "concentration and attention levels" at the Games. His previous loss to Tiafoe had come days after he was beaten in a heartbreaking five-setterby top-ranked Novak Djokovic in the French Open final. Argentina"s Diego Schwartzman, who took the court after play resumed in the afternoon, also advanced with a 6-4 7-5 win over Tomas Machac of the Czech Republic. "OPEN WOMEN"S DRAW" The women"s draw was thrown wide open after four-time Grand Slam champion and world number two Osaka was downed 6-1 6-4 by Marketa Vondrousova of the Czech Republic. Osaka, who returned to competition this week after withdrawing from the French Open in May citing mental health issues, never found her stride against the 42nd-ranked Vondrousova, attributing the loss to the pressure she faced as one of Japan"s most prominent Olympic ambassadors. read more Vondrousova, who had never played Osaka before, described the match as one of the biggest victories of her career. "Of course it"s one of the biggest. Naomi is a great player, she has so many Grand Slams, so I knew it would be a tough match. But I"m just very happy with my play," the 22-year-old said. Osaka"s defeat came after top-ranked Australian Ash Barty and world number three Aryna Sabalenka were put out in earlier rounds. French Open champion Barbora Krejcikova was also beaten on Tuesday, losing 1-6 6-2 6-3 to Switzerland"s Belinda Bencic. "It"s very open now," said Vondrousova, who next meets Spain"s Paula Badosa. "I think every girl is playing really well. Now it"s the quarter-final, so we"ll see."