Roddick needed five match points to get the better of Simon. The American will play No. 13 Jo-Wilfried Tsonga , who beat No. 3 Novak Djokovic 6-4, 1-6, 6-3 in a rematch of the Australian Open final.
The Masters Cup in Shanghai features the top eight players in the ATP race. Eleventh-seeded James Blake of the United States remains in contention after edging Philipp Kohlschreiber 6-4, 6-4.
Roddick said he had to dictate the pace against Simon.
"It wasn't going to do me any good to play patty-cake back and forth with him today,'' Roddick said. "I'm not as quick as he is and I'm not as consistent as he is. It actually made for a pretty simple game plan. "
Simon was powerless to attack Roddick's first serve, winning only four points on it. Simon also missed four chances to break Roddick in the second set.
Roddick clinched the match when Simon went for a forehand winner and the ball clipped the net and bounced out.
The eighth-seeded Nalbandian broke Del Potro's serve twice in the first set and three times in the second to defeat his Argentine compatriot for the second time in less than a week after winning in the Swiss Indoors semifinals on Saturday.
"I think he was a little tired, particularly in the second set,'' Nalbandian said. "I think I played well all the match and it was a very good performance.''
Del Potro, who had beaten Nalbandian in the third round of the Madrid Masters two weeks ago, won only 13 points in the second set as Nalbandian repeatedly attacked his weak second serve.
Nalbandian will face either No. 4 Andy Murray or No. 15 Fernando Verdasco, who plays later.
Later, top-ranked Rafael Nadal meets No. 16 Gael Monfils and No. 2 Roger Federer plays Marin Cilic.
Nalbandian through to 3rd round at Paris Masters
McClaren Wants A Race To Victory
