The six-time champion ensured he will not have to rely on other results with a 6-4 7-6 (7-5) win.
Federer has had two straight-sets wins and plays Juan Martin del Potro in his last Group B match on Saturday.
Federer has made typically serene progress at London's O2 Arena and, despite Ferrer's recent winning streak of 11 matches, a head-to-head record of 0-13 suggested he was always likely to struggle against the Swiss star.
The Spaniard needed to take every opportunity that came his way, but failed to capitalise as Federer offered up unforced errors and numerous second serves in the opening stages.
After breaking serve at the first opportunity, Federer faced seven break points across three games before a backhand flew long to let Ferrer back into the set at 3-3.
The fourth seed was scrapping hard to stay in touch but Federer has shown he has an extra gear in their previous meetings, and he found it once again.
A brilliant flurry of forehands saw the Swiss accelerate past Ferrer with a love break in game 10 that secured the set, and the 17-time Grand Slam champion steadily improved his game from then on.
A first-serve percentage that had slumped as low as 34% in the first set hit the mid-60s in the second, helping to fend off two more break points as Ferrer battled to keep his winning streak going.
It came down to a tie-break - only the second between the pair - and Federer's now functioning serve proved the dominant shot as he converted his third match point after one hour and 48 minutes.
Source: BBC
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