Basel - Two-time champion Juan Martin del Potro advanced to the Swiss Indoors quarter-finals by beating fifth-seeded David Goffin of Belgium 7-5, 6-3 on Thursday.
Del Potro, who won the Stockholm indoor title on Sunday, extended his winning streak to seven matches - six against players higher in the standings than the 42nd-ranked Argentine player.
Goffin, who beat Del Potro at the Shanghai Masters two weeks ago, struggled with his second serves and was broken once in each set.
Del Potro won back-to-back titles at Basel in 2012 and 2013, but missed the last two editions due to left wrist injuries that dropped his ranking sharply.
The 28-year-old will play No 5 Kei Nishikori of Japan on Friday.
Fourth-seeded Marin Cilic also advanced on Thursday in a 6-0, 7-6 (4) win over Pablo Carreno Busta.
Cilic lost only five points in the first set against the 32nd-ranked Spaniard.
The second set went with serve, and Cilic trailed in the tie breaker before reeling off seven straight points.
Cilic next plays Marcel Granollers of Spain, who beat eighth-seeded Jack Sock of the United States 6-3, 6-1.
Earlier, Gilles Muller of Luxembourg earned a spot in the last eight against Federico Delbonis of Argentina.
Muller beat Florian Mayer of Germany 6-3, 6-2.
Results on the fourth day of the Swiss ATP Indoors on Thursday (x denotes seeded player):
Marin Cilic (CRO x4) bt Pablo Carreno Busta (ESP) 6-0, 7-6 (7/4)
Marcel Granollers (ESP) bt Jack Sock (USA x8) 6-3, 6-1
Gilles Muller (LUX) bt Florian Mayer (GER) 6-3, 6-2
Juan Martin del Potro (ARG) bt David Goffin (BEL x5) 7-5, 6-3