(Updates after Federer match)
May 16 (Reuters) - Defending champion Rafa Nadal recovered from losing the opening five games against Ernests Gulbis to book his place in the quarter-finals of the Rome Masters on Thursday.
The Spaniard, looking for his seventh Rome title, was shaken by a ferocious onslaught from Gulbis, losing the first set in 34 minutes, but recovered to win 1-6 7-5 6-4.
World number one Novak Djokovic had a far more comfortable workout, beating Ukraine's Alexandr Dolgopolov 6-1 6-4 but seventh seed Juan Martin del Potro, who met the Pope the previous day, came back to earth with a surprise defeat by Frenchman Benoit Paire.
Paire's 6-4 7-6 victory was the sole highlight for French players, however, with Gilles Simon thrashed by second seed Roger Federer after ninth seed Richard Gasquet lost a stormy encounter with Polish firebrand Jerzy Janowicz.
In the women's tournament world number one Serena Williams tied her longest career winning streak with a 6-0 6-1 rout of Dominika Cibulkova, taking her run of victories to 21.
Williams, who served five aces in one game, looks unstoppable but issued a few words of caution.
"Nothing is ever perfect - I learned that last year when I felt perfect," Williams said, referring to her first-round loss at last year's French Open. "I am still in the danger zone."
She was joined in the quarter-finals by Maria Sharapova who breezed past Sloane Stephens 6-2 6-1 while Serbia's Jelena Jankovic, a former Rome champion showing signs of a return to her best form, beat China's fifth seed Li Na 7-6 7-5.
Nadal will face compatriot David Ferrer in the last eight after he was given a walkover by Germany's Philipp Kohlschreiber who pulled out suffering from vertigo.
It was Nadal who was feeling giddy at the Foro Italico though as Gulbis, playing like the top-tenner many think he could be rather than the world's number 46, hit the lines.
Gulbis struck 16 winners in taking the first set and had chances to move ahead in the second before Nadal began to turn things around, breaking to lead 5-3.
However, Gulbis broke back and then saved three set points from 0-40 in the next game, reeling off six consecutive points.
Nadal recovered to level the match and then forged 4-2 ahead in the third, only for Gulbis to pull level again.
Gulbis finally ran out of firepower when serving to stay in the match at 4-5, allowing Nadal to clinch victory.
Djokovic will face Czech Tomas Berdych after the Czech sixth seed defeated South African Kevin Anderson 7-5 6-2.
Janowicz, who will meet Federer in the last eight, courted controversy when he pointed at the wrong ball mark after asking the umpire to check where a winning Gasquet volley had landed on a break point early in the second set.
His roars and gesticulations at important moments also attracted jeers from the crowd, although there was no doubting the quality of his game as he claimed a 3-6 7-6 6-4 win. (Writing by Clare Fallon in London; Editing by Pritha Sarkar and Alison Wildey)