Both titles have grown sales by more than 20% in the past year, with Grazia's weekly sales leaping ahead of the monthly circulation of Elle, Easy Living and InStyle, after a particularly impressive second half.
InStyle in particular appears to be the biggest victim of Grazia's launch, with the biggest period-on-period drop in the fashion market - its first fall since launching in the UK.
The resilience of the monthlies is impressive, with more than 250,000 more copies in circulation than in the first half of last year, and market leader Glamour still showing a marginal increase after more than five years at the top.
With many editors swapping titles, magazines have benefited from a fresh focus. For example, Jessica Burley, NatMags' managing director, admits being apprehensive at new editor Louise Chunn's proposal to put Jamie Oliver on the front of Good Housekeeping - its first male cover star in decades. But the gamble paid off, as did an overtly Christmas-decked cover that led to the biggest sale for the magazine's January issue in five years.
David Barnett, press account director at Universal McCann, is encouraged by publishers' efforts to strengthen their big titles. "Everyone's been spending on marketing and overhauling their editorial," he noted.
Abby Carvosso, publisher of Grazia, said the title's sponsorship of the London Fashion Weekend was a turning point for its brand name.
"It's taken time for advertisers to get used to (the magazine) and for us to prove that, unlike many other weeklies, it doesn't have peaks and troughs because the glossy audience wants fashion first and foremost. We have now got a core group of people forming the habit of buying it."
Questions still hang over the younger end of the market. Essential Publishing closed Real this month, weeks after changing its frequency from fortnightly to monthly, and Emap's New Woman and IPC's Essentials have had makeovers.
Julie Harris, general manager of women's titles at Hachette Filipacchi, said that, even with these relaunches, publishers are failing to adapt the titles to today's audiences.
"There's still a place for young monthlies, but they've gone off the boil editorially because they're competing with weeklies," she said. "You need to indulge readers with good monthly content that weeklies can't possibly cover."
By contrast, upmarket fashion magazines are all performing exceptionally strongly and the challenge now for publishers is to make them work online alongside the print editions. Hachette has launched the first website for Elle, while IPC is launching InStyle's first UK site in April.
It has taken years to get these sites off the ground and their emergence appears to be proof that these magazines now feel confident enough to extend their brands online.