Zverev celebrates first victory over Nadal
Defending champion Alexander Zverev has inflicted a blow on Rafael Nadal's year-end No.1 chances after defeating the Spaniard in straight sets at the ATP Finals.
Zverev, who had only won one match against a top-10 player this year before facing Nadal, produced an emphatic service display throughout his 6-2 6-4 victory, claiming his first ever win over the king of clay on his sixth attempt.
Stefanos Tsitsipas made it sixth time lucky with a surprise win over fellow ATP Finals debutant Daniil Medvedev in their opening round-robin match.
Sixth seed Tsitsipas had a record of played five, lost five against Medvedev, who was this year's US Open runner-up having enjoyed a stellar breakthrough season.
But the 21-year-old from Greece found London's 02 Arena to his liking as he ground out a 7-6 (5) 6-4 victory.
Russian Medvedev (23) has won more matches than any other player this year, his total of 59 victories being five more than Novak Djokovic has managed.
But he could not manufacture a single break point against the dogged Tsitsipas, who secured the only break at 5-4 in the second set before serving out.
Tsitsipas said: "It was one of the toughest, most important victories of my career so far.
"I gave myself a big boost, I kept believing, kept fighting, believing in myself, and that last game was one of the toughest.
"It's such a relief. It's not easy coming in knowing you lost five before, but I made a deal with myself to keep trying. I had great support, there are Greek flags everywhere, it feels like I'm in Athens. I had goosebumps."
On Sunday, Roger Federer's return to London fell flat. The six-time champion was beaten in straight sets, 7-5 7-5.
Federer said: "I thought the match was actually pretty even for a long period of time. I had my chances, I felt. I didn't feel like I was outplayed or anything. Just maybe that first-round hiccup.
"It's a normal tournament from here on forward. I'm not allowed to lose anymore. That's how it is every week of the year for the last 20 years, so from that standpoint there is nothing new there."