The chances for Mardy Fish to play in his first ATP season finale may have been badly compromised as the American suffered a hamstring injury in his first round match at the Swiss Indoors here on Tuesday.
Fish was leading James Blake 1-0 in the opening set when he was forced to retire.
"I didn't see anything. He suddenly started grabbing at his hamstring and had to call the trainer," said Blake.
"It's worrisome that it appeared to happen on one shot and not over the course of a few days.
"But Mardy is doing the cautious thing, he's gone for an MRI and we will have to see."
The retirement, after just 11 minutes on court, could mean the end of Fish's hopes of winning a place in the World Tour Finals in London from November 20.
The 29-year-old stands provisional eighth in the race for the eight-man field, with three places still open.
The definitive line-up will be determined this week and next as the ATP regular season draws to a close.
Fish, the number one American after replacing Andy Roddick, had been on a tear, winning 20 of 26 matches since July.
He still has 500 points of breathing space in the race between himself and provisional ninth-placed Spaniard Nicolas Almagro.
Blake has been on an indoor roll for the last few weeks, winning five of seven matches and reaching the Stockholm semi-finals.
The one-time world No 4, now ranked 60th, has climbed back from 173rd this season but had to qualify into Basel, the first time he's played the qualifiers since the 2005 French Open.
The pair of American friends and practise partners were meeting for the first time outside the US, with the series now level at three wins each.
Blake will next play Russian Mikhail Kukushkin, who got into the field after Austrian Jurgen Melzer pulled out with a back injury.
Two Swiss booked victories on the home court, with Stanislas Wawrinka leading the way with his defeat of Croatian Ivan Dodig 6-4, 6-4. Qualifier Michael Lammer put out Russian Mikhail Youzhny 6-4, 6-3.
Swiss number two Wawrinka levelled his Basel record at 7-7 after reaching the semi-finals in 2006 and quarter-finals in 2009.
He will next face the winner from second seed Andy Murray or Dutchman Robin Haase.