Lovtsova's inclusion forced officials to hastily reissue the starting list for the event, in a sign of the turmoil prompted by a rush of last-minute appeals by banned Russian competitors.
Breaststroker Yulia Efimova suddenly appeared in the start lists for the 100 metres, whose heats take place on Sunday, and Wednesday's 200m, the event in which she won an Olympic bronze medal in London in 2012. On Friday her name had been absent from the field for both races.
The build-up to the Rio Games has been overshadowed by revelations of widespread state-sponsored doping in Russia. Under guidelines laid down by the International Olympic Committee, but left ultimately to the discretion of individual sports federations, Russians with past doping convictions were ineligible to compete.
But a number have successfully appealed to the Court of Arbitration for Sport, arguing that once their bans have been served, they should not be punished a second time for the same offence.
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Lovtsova came eighth and last in the final heat of the 100m butterfly, won by world champion Sarah Sjostrom of Sweden. She left without speaking to reporters.
Efimova is likely to mount a more powerful challenge in the breaststroke events.
On Friday, after learning of her reinstatement, she posted a picture of herself on Instagram, looking surprised and tearful, with the caption: "I'm going to the Olympics. I couldn't be more proud and relieved."
(Reporting by Mark Trevelyan, editing by Ed Osmond and Tom Brown)
