Andy Murray says he is now serving as hard as he did when he was in his mid-20s, which is "good news" for his body as he rebuilds his injury-hit career.
The Scot, 33, reached the semi-finals of the Battle of Brits last week in his first event for seven months and was "pleasantly surprised" by his tennis.
"I just wasn't able to sustain it for long enough," he said in an interview for this weekend's Andy Murray's Greatest Hits programmes on BBC One.
"At times I played really well."
Former world number one Murray had career-saving hip surgery in January 2019 after pain had dogged him for a number of years.