Sevens heavyweights Samoa, Fiji, South Africa and New Zealand all went unbeaten on Day One in Las Vegas after disappointing displays in Wellington last week.
On a fascinating day, which saw a stark reversal in fortunes from last weekend, Wellington Cup champions England lost both of their matches to Fiji and Portugal, while fellow Sevens finalists Kenya lost to Canada and only narrowly beat Uruguay.
However, the Cup quarterfinals remain wide open with the final pool games still to be played on Saturday.
All 16 teams are still capable of qualifying for the Cup quarterfinals on Day Two. If three teams are level on competition points at the end of the pool stages, the team with the highest points difference will go through to the Cup quarterfinals.
We look at all of the Day One action:
(England, Scotland, Fiji, Portugal)
Fiji, who will be without both their suspended captain Setefano Cakau and Manueli Lagai for the remainder of the tournament, won both of their opening matches, narrowly holding on to a 14-12 victory against Scotland, with Russell Weir narrowly missing a last-minute penalty drop-goal.
England, last week's champions, lost both of their opening matches, against Fiji 26-17 with two tries from Samisoni Viriviri and Portugal, 21-5.
Scotland beat Portugal in their first match, with Mark Robertson and Michael Fedo scoring the tries and Colin Gregor's touchline conversion proving decisive. All three teams, however, are still in with a chance of reaching the Cup competition depending on results on day two.
England 17-26 Fiji
Scotland 12-10 Portugal
England 5-21 Portugal
Scotland 12-14 Fiji
(Kenya, South Africa, Canada, Uruguay)
South Africa became the second side to end day one with a 100% record, adding a 17-5 victory against Canada to an impressive 45-0 victory against the only non-core side competing in Las Vegas, Uruguay.
Canada defeated Kenya early on, which meant both of the Sevens finalists lost their opening matches. Sean Duke crossed for his 22nd try of the series with Nanyak Dala and Jeffery Hassler scoring Canada's other tries.
Mike Friday's Kenyans bounced back from their opening defeat, riding their luck to come from 7-5 down and inflict a second defeat on Uruguay, Biko Adema benefiting from smart work by Oscar Ayodi.
Kenya 12-19 Canada
South Africa 45-0 Uruguay
Kenya 12-7 Uruguay
South Africa 17-5 Canada
(New Zealand, Argentina, France, Wales)
New Zealand also won both of their two matches on day one, conceding just five points in the process. Belgium Tuatagaloa helped the reigning Series champions beat France 21-0 with two of their three tries before they beat Wales 40-5 with Ben Lam crossing for a hat-trick.
Reigning Rugby World Cup Sevens champions Wales beat Argentina in a rematch of that 2009 final in Dubai 17-7 in their first match of the USA Sevens.
Javier Rojas secured a 14-12 victory for Argentina against France, Los Pumas' first win of the day, and Les Bleus' second defeat. Both sides can still reach the Cup quarterfinals.
New Zealand 21-0 France
Argentina 7-17 Wales
New Zealand 40-5 Wales
Argentina 14-12 France
(Samoa, Australia, Spain, USA)
Defending champions Samoa maintained a 100% record on day one, beating hosts USA and Spain. They came from 12-0 behind to beat Spain 19-12, with captain Afa Aiono scoring the winning try under the posts to the delight of the crowd at the Sam Boyd Stadium, before Levi Asifa'amatala and Reupena Levasa scored the tries against USA.
Spain bounced back to defeat Australia 24-14 to bring an end to an impressive run from Michael O'Connor's side, who had won nine of their previous 10 matches. The hosts USA lost their opening match to Australia 26-7, although they too can still make it to the Cup tomorrow if they beat Spain and points difference goes their way.
Samoa 19-12 Spain
Australia 26-7 USA
Australia 14-24 Spain
Samoa 12-7 USA