Top seed Jack Sock battled into the US Men's Clay Court Championship semi-finals, but John Isner was outlasted in a marathon clash.

Sock, finalist in Houston the past two years and winner in 2015, got past Feliciano Lopez 7-6 (8-6) 1-6 6-4 in two hours, 14 minutes on Friday.

The American broke twice in the third set at the ATP 250 event, reaching a fourth semi-final of the year.

Sock holds a 20-4 win-loss record this year, including titles in Auckland and Delray Beach.

Awaiting Sock in the last four is Steve Johnson, the American fourth seed having cruised past Fernando Verdasco 6-2 6-4.

Isner, the second seed and 2013 champion, was edged by fellow American and wildcard Ernesto Escobedo 7-6 (8-6) 6-7 (6-8) 7-6 (7-5).

It took just over three hours for 20-year-old Escobedo to get through, with only two breaks of serve throughout the encounter.

Isner sent down 35 aces – including 17 in the final set – but was edged in the tie-break.

Escobedo will face Thomaz Bellucci in the semi-finals after the Brazilian eighth seed beat two-time runner-up Sam Querrey 6-4 3-6 6-3.