I had a real neat, deep-stacked heads up match going on two tables
for several hours yesterday. I felt like we were very evenly matched,
and indeed I got off to an early lead, eventually lost a 700BB pot with
two pair vs. a turned flush in a four-bet pot and lost a few more big
pots to fall way behind, then got back into the lead again when I set
over set him for 700BB. At that point he started tilting a bit, and I
felt like I had an advantage for that reason. Here's one of the key
hands that helped swing the momentum back my way:

Full Tilt No-Limit Hold'em, $20.00 BB (2 handed) - Full-Tilt Converter Tool from FlopTurnRiver.com

Hero (SB) ($4218.50)
BB ($6489)

Preflop: Hero is SB with 5, 5
Hero bets $60, BB calls $40

Flop: ($120) 6, 5, 7 (2 players)
BB checks, Hero bets $77, BB calls $77

Turn: ($274) J (2 players)
BB checks, Hero bets $222, BB calls $222

River: ($718) 10 (2 players)
BB checks, Hero bets $499, BB raises to $1950, Hero calls $1451

Total pot: $4618 | Rake: $0.50

Results:
Hero had 5, 5 (three of a kind, fives).
BB had 7, A (one pair, sevens).
Outcome: Hero won $4617.50

In some ways, this is a good spot for him to turn his hand into a
bluff. He has some bluff-catching value with a call, but I'm also
value-betting quite a few better hands than his, probably AT+. The only
problem is that he doesn't represent very much. I'm not worried about
him doing this for value with less than a flush, and I think he's
usually playing a flush draw faster than this from out of position.
Even something like Ac6c, which with both the nut flush draw and a pair
is probably the ideal hand for a line like this, may well be raising
the flop (or 3-betting pre-flop, which he did quite liberally).

Still, it's a pretty sophisticated play on his part, and opponents
capable of moves like this are very difficult to play against. Knowing
that he's capable of turning a made hand into a check-raise bluff, I
have to constrain my value betting a bit on future rivers due to the
risk of opening myself up to a bluff that I can't call.