Murray's scrawled message on the television camera - which is customary for the winner - indicated his satisfaction at extending what is expected to be the two-time champion's final appearance in Miami.

The former world number one, who turns 37 in May, recently said he does not "plan on playing much past the summer" when pressed about his retirement plans.

Murray has been candid about his struggles in recent months and looked to be heading to another defeat against former world number six Berrettini, who was playing his first ATP Tour match in almost seven months.

Berrettini, whose own career has been stalled by a succession of long-term injuries, demonstrated he has not lost his strong service game by landing 78% of his typically-hefty first serves in an impressive first set.

Three-time major champion Murray, rarely one to roll over, improved considerably in the second set and turned the match around.

More proactive in the points and making fewer unforced errors, the Scot was rewarded by taking his third break point of the game for a 4-2 lead.

Then came a dramatic and concerning moment when Berrettini, serving to stay in the set at 5-2, wobbled as he was preparing to play.

The 27-year-old needed treatment from a doctor for dizziness before being able to continue, but also took a long break off the court before the deciding set.

With Berrettini still looking uncomfortable, Murray maintained his intensity to break at the fourth time of asking in a 12-minute opening game and it was enough to secure a much-needed success.

His celebration was respectfully reserved given the circumstances and he made sure his first act was to ask Berrettini how he felt when they shook hands at the net.