Carlos Alcaraz had never lost to Casper Ruud previously - winning all four matches and nine of the 10 sets.
Heading into their match at the ATP Finals, Ruud had lost to the likes of Henrique Rocha and Aleksandar Vukic and appeared horribly out of form.
But the Norwegian showed the form that has made him a three-time major finalist to blitz Alcaraz 6-1 7-5 in a huge upset at the season-ending event in Turin.
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The courts at the Inalpi Arena weren't expected to suit Alcaraz, but no one expected the result of the first set.
Ruud saved all five break points on his serve and converted the only two against Alcaraz to dominate the world no.3.
Alcaraz provided resistance in the second set, breaking in the sixth game to go ahead 4-2.
The Spaniard then failed to serve out the set at 5-3 and was broken again in his next service game, with Ruud closing the match out at 7-5.
Alcaraz will hope to bounce back with round robin matches against Alexander Zverev and Andrey Rublev, while Ruud will play the same opponents hoping to secure what previously seemed an unlikely semi-final berth.
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