Andy Murray began his ATP World Tour Finals campaign with a convincing win over Spain's David Ferrer in London.
The Briton, 28, won 6-4 6-4 at the O2 Arena and will next play the winner of the evening match between Stan Wawrinka and Rafael Nadal.
Another victory on Wednesday would ensure Murray ends the year as world number two for the first time.
The Scot's season will continue next week as Great Britain take on Belgium in the Davis Cup final.
"It was a tough match with a lot of long rallies," said Murray. "He fought hard right to the end and made it extremely difficult.
"He didn't serve as well as he can and I played a bit better at the end of both sets, and that got me the win."
After some uncertainty over whether Murray would sacrifice his place in London to remain healthy for the showpiece in Belgium on 27-29 November, the Scot was in committed mood once he stepped out into the O2 Arena for his first round-robin match.
He spent several days last week practising on clay for Britain's first Davis Cup final since 1978, but there was little sign of rustiness on the switch back to an indoor hard court.
Four break points slipped by in the first eight games before Murray finally broke through, thanks to a fine volley and a Ferrer double fault.
The seventh seed, 33, was misfiring, making just 47% of first serves, and he was thankful for a loose Murray service game at the start of the second set.
Ferrer was soon battling to keep Murray at bay once again, however, succumbing in game six as the Scot levelled.
Serving to stay in the match, an eighth double fault saw Ferrer go 0-30 down. Then presented with a first match point a bouncing Murray leapt to put away a smash at the second attempt.
The victory gives him an early lead in Group Ilie Nastase, with two players to qualify for the semi-finals on Saturday.
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