Having already claimed the world No.1 ranking from Novak Djokovic, Andy Murray has beaten American John Isner 6-3, 6-7, 6-4 to win the Paris Masters.
The win is Murray's fourth consecutive title, taking his career tally to 43.
He has now also won 14 Masters title overall.
Murray, 29, becomes the second oldest player to debut at No.1, behind Australian John Newcombe in June 1974 and he is the first British player to ever be world No.1.
Murray officially claimed the world's top ranking on Sunday AEDT, when he received a walkover in the Paris Master semifinal, with Milos Raonic pulling out the clash with injury.
“This morning I had trouble waking up and getting out of bed. So I went to sort of clear any serious diagnosis,” Raonic said.
“Did some tests. Did an MRI half an hour ago, let’s say. They found that I have a tear, Grade 1 tear in the right quad.
“Unfortunately, I’m not able to compete against Andy today in the second semi-final.”
Djokovic has earlier been knocked out of the tournament, opening the door for Murray to leap frog his Serbian rival.