American Roddick is currently ranked 11th in the world
Andy Roddick is through to the second round of the Shanghai Masters after his opponent Philipp Kohlschreiber retired because of illness.
The 10th seed was leading 6-3 2-1 when the German, who had just dropped his serve, decided he could not continue.
Roddick, 28, said: "We've all been on the bad side of having to pull out of a match, it's not fun. But I hit the ball well in what we did play today."
The American faces Spain's Guillermo Garcia-Lopez in the next round.
Roddick reached the quarter-finals of the Japan Open last week - his first tournament since a disappointing second-round exit at the US Open.
The world number 11 said: "I don't feel like I'm fighting myself out there. After the Open, I was able to train the way I want to for the first time in a while."
Roddick's fellow-American John Isner also reached the second round in Shanghai after saving match points in two tie-breakers against Polish qualifier Lukasz Kubot.
Isner, best known for his marathon encounter with Nicolas Mahut at this year's Wimbledon, eventually triumphed 4-6 7-6 (11-9) 7-6 (9-7) and will face Roger Federer in round three.
Chinese wild card Bai Yan, ranked 465th in the world, set up a meeting with Andy Murray after upsetting world number 30 Radek Stepanek 6-4 6-4.
It was Bai's first win on the main tour and he admitted: "Murray is a very talented player - much better than me. I hope to learn from him in our match.
"It's my first time in a top level tournament, so it's really an unexpected performance for me."
Meanwhile, 11th seed David Ferrer bounced back after losing last week's China Open to defeat Michael Llodra of France 7-6 (7-2) 6-1.
In Tuesday's only second round match, Wimbledon runner-up Tomas Berdych overcame a stomach problem to see off Spain's Tommy Robredo 6-0 6-4.
The seventh-seeded Czech said: "I felt quite weak so I was trying to play a little more aggressively, play really risky shots just to try to hang in there. I was really lucky - I don't know what could have happened if I lost the second set."