London, Nov 19 : Daniil Medvedev has guaranteed his place in the semi-finals at the ongoing ATP Finals as the Russia fourth seed stunned world number one Novak Djokovic 6-3, 6-3 in their latest encounter. Medvedev, who lost all three group matches in last year's season finale, had claimed his first victory in the tournament two days ago by beating 2018 champion Alexander Zverev in straight sets, reports Xinhua news agency. Facing five-time champion Djokovic in the second round in Group Tokyo 1970, the 24-year-old Russian on Wednesday went unbroken while forcing the Serbian ace to make mistakes in his own service games. "To be completely honest, I am sure he didn't play his best today," said Medvedev. "But it happens to everybody. I always say the Big Three are the champions because it happens less for them than for other players. It is still tough to beat them, even on their bad days. So I am pleased about the win." "He was just better, no question about it," admitted Djokovic. "I just struggled to find the right rhythm for 15 minutes or so. He used it, and then he held his serve all the way till the end. I Earlier, Zverev kept his semi-final hopes alive as the German giant beat Argentine debutant Diego Schwartzman 6-3, 4-6 and 6-3. "I am happy to give myself a chance to go to the semi-finals and I am excited for Friday," said Zverev. Zverev will lock horns with Djokovic on Friday. /IANS