Rafael Nadal reacts during the match against Alexander Zverev at the ATP World Tour Finals on Monday in London. Photo: VCG

Rafael Nadal refused to make excuses after being outplayed by defending champion Alexander Zverev at the ATP Finals on Monday as Stefanos Tsitsipas got his campaign off to a flying start.

The Spanish top seed, who has never won the season-­ending event, came into the tournament at London's O2 Arena under an injury cloud and was well short of his imperious best as he went down 6-2, 6-4.

In the earlier round-robin match in Group Andre Agassi, sixth seed Tsitsipas beat Daniil Medvedev 7-6 (7/5), 6-4, admitting their spiky relationship had made his job tougher.

Nadal is locked in a battle with Novak Djokovic to finish as the year-end No.1 but was not serving flat out in practice last week due to an abdominal strain that forced him to pull out of the Paris Masters at the semifinal stage.

The 33-year-old led Germany's Zverev 5-0 in head-to-head contests coming into the match but was uncharacteristically sloppy in the first set, conceding two breaks of serve.

Buoyed by his dominant start, Zverev, seeded seventh, broke in the first game of the second set to take an iron grip on the match.

Nadal dug deep, urging himself on but his 22-year-old opponent proved too strong. He did not concede a single break point in the entire match.

Zverev hit a total of 26 winners - double his opponent's tally - and Nadal managed just three forehand winners in the entire match.

"The physical issue was not an excuse at all," said the Spanish 19-time Grand Slam champion, who said he did not feel any pain from his abdominal injury. "The only excuse is I was not good enough tonight."

Nadal faces a tough task now to qualify for the semifinals from the group phase, with matches still to come against Tsitsipas and Medvedev.

The Spaniard has qualified for the year-end championships for 15 years in a row but has only made nine appearances due to injuries.

Earlier, Tsitsipas made light of a 5-0 losing record against his Russia's Medvedev, edging a first-set tiebreaker and breaking late in the second set to seal the match. The two players have had a rocky relationship.

Tensions flared between them in Miami last year and Tsitsipas recently labelled Medvedev's way of winning as "boring" after defeat against the Russian in Shanghai.

The Greek player said Medvedev had got "into my head" in Miami - he said the row had centerd on a demand from the Russian for an apology over a net cord. "I mean, our chemistry definitely isn't the best that you can find on the Tour," said Tsitsipas, who added that Monday's match had been tougher because he was so desperate to win.

Djokovic launched his bid for a sixth ATP Finals title with a comfortable win against Matteo Berrettini on Sunday but Roger Federer slipped to a straight-set defeat against Dominic Thiem.