With strength across all channels, we delivered comparable store sales growth of 24.7%, and margin expansion of 478 basis points versus the prior year.
On a two-year stack, our comp sales growth was 15.4%.
Adjusted diluted earnings per share of $3.34 represented an all-time quarterly high for AAP, and improved more than 230% compared to Q1 2020.
Free cash flow of $259 million was up significantly versus the prior year, and we returned over $203 million to our shareholders through a combination of share repurchases and our quarterly cash dividend.
Geographically, all eight regions posted over 20% growth.
As the country began to reopen later in the quarter, Professional came on strong, resulting in Pro growth of over 20% in Q1, with continued momentum into Q2.
Today, we're extremely excited to announce that we're adding 29 new independent locations to the Pacific Northwest to the Carquest family, the single largest convergence in our history.
Baxter Auto Parts announced that they will bring over 80 years of automotive aftermarket experience and strong customer relationships to the Carquest banner.
We now have over 13,000 North American members, and we'll continue to leverage TechNet to differentiate our Pro offering and build loyalty.
According to syndicated data, an estimated 4 million new DIY buyers were added.
Spend per buyer for 2020 grew close to 9%, led by online spend per buyer.
In Q1, this helped drive growth in our VIP members by approximately 14% and our Elite members by 30%.
We continue to strengthen our online experience on desktop, mobile and with our app, which recently crossed nearly 1.3 million downloads.
This enables DIYers to find the right part from our industry-leading assortment, order it online, and either pick it up in one of our stores within 30 minutes or have it delivered in three hours or less.
We're targeting between 100 to 115 new stores in 2021.
In total, our category management initiatives are currently on track to deliver up to 200 basis points of margin expansion through 2023.
In terms of cross-banner replenishment, or CBR, we've converted over 70% of stores to date and expect to complete the remaining stores we originally planned by the end of Q3.
More importantly, CBR will complete the integration of the Advance and Carquest supply chains and enables us to service our approximately 4,800 corporate Advance stores and 1,300 independent Carquest stores from a single supply chain.
We've now increased sales per store for three straight years, and we're on track to get to our target of $1.8 million average sales per store by 2023.
We delivered a 9% reduction in our total recordable injury rate compared to the previous year, and reduced our lost-time injury rate 2%.
This will result in savings of approximately $30 million in SG&A, which will be realized over the next 12 months.
In Q1, our net sales increased 23.4% to $3.3 billion.
Adjusted gross profit margin expanded 91 basis points to 44.8% as a result of improvement throughout gross margin, including supply chain, net pricing, channel mix and material cost optimization.
Our Q1 adjusted SG&A expense was $1.2 billion.
On a rate basis, this represented 35.8% of net sales, which improved 387 basis points compared to one year ago.
Related to the increased COVID-19 cases we saw late in 2020 and early 2021, we incurred approximately $16 million in COVID-19 cost during the quarter, which is flat to the prior year.
Our adjusted operating income increased from $113 million last year to $299 million.
On a rate basis, our adjusted OI margin expanded by 478 basis points to 9%.
Finally, our adjusted diluted earnings per share was $3.34, up from $1.00 a year ago.
Our free cash flow for the quarter was $259 million, an increase of $330 million compared to last year.
Our AP ratio improved by nearly 1,000 basis points to 84%, the highest we've achieved since the GPI acquisition.
In the quarter, we spent $71 million in capital expenditures versus $83 million in the prior year quarter.
During Q1, we returned more than $200 million to our shareholders through the repurchase of 1.1 million shares and our quarterly cash dividend.
We expect to be within our 2021 share repurchase guidance of $300 million to $500 million.
For these reasons, we're raising our comp sales guidance to up 4% to 6%.
As a result of our top-line strength and current cost assumptions, we're updating our adjusted OI margin range to be between 9% and 9.2%.
Our guide for comp sales is now up 3 full points, and our adjusted OI margin rate is now up 30 basis points compared to our initial guidance provided in February.

Summary:
With strength across all channels, we delivered comparable store sales growth of 24.7%, and margin expansion of 478 basis points versus the prior year.
Adjusted diluted earnings per share of $3.34 represented an all-time quarterly high for AAP, and improved more than 230% compared to Q1 2020.
In Q1, our net sales increased 23.4% to $3.3 billion.
Finally, our adjusted diluted earnings per share was $3.34, up from $1.00 a year ago.