In his first match since the US Open, del Potro was upset by world No. 189 Édouard Roger-Vasselin in straight sets at the Rakuten Japan Open Tennis Championships in Tokyo. He then lost his second straight match to Melzer in the second round at the Masters 1000 event in Shanghai, retiring while trailing in the second set. This retirement caused concerns over the length of the tennis season. He had to retire again in the Paris Masters quarterfinals when down 0–4 to Radek Štěpánek due to an abdominal injury. In November, del Potro competed in the ATP World Tour Finals, where he lost his first round-robin match against Andy Murray, but he managed to defeat Fernando Verdasco in his second match to keep his hopes alive. After defeating Roger Federer in the following match, he qualified for the semifinals, ousting Murray by the slimmest possible margin of one game. He defeated Robin Söderling in the semifinals, before losing to Nikolay Davydenko in the final. Del Potro finished 2009 as the youngest player in the top 10, top-ranked Argentine, and highest-ranked South American for the second consecutive year.